Allow incident severity updates to trigger issue webhooks

What does this MR do?

Related issue: #333774 (closed)

This MR allows changes to an incident's severity to trigger the issue webhooks.

As a side-effect, I also removed the severity system note on incident creation, as the user isn't actually in control of the severity that the incident is initialized with. Now we only create a severity-related system note when the severity is changed by a user.

Testing

1. Setup a webhook receiver https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#example-webhook-receiver
2. Setup a webhook which pushes Issues events at Settings > Webhooks
3. Create a new incident (New Issue w/ type=Incident || Monitor > Incidents)
   • See the request come into the receiver: severity should be an attribute in the body
   • A system note should not be created for the severity
4. Update the severity of the incident
   • See the request come into the receiver: severity should be present under the "changes" as well as in the body
   • A system note should be created for the severity change
5. Change another attribute on the incident (like milestone or labels)
   • See the request come into the receiver: severity should not be present under "changes"

Other issuables/interactions:

• For non-incident issues, severity should be present in the webhook payload with a constant value of "unknown"
• For MRs, severity should not be present in the webhook payload
• For incidents created from alerts, the severity should be set to match the alert's severity at creation-time

UI interactions

About to create incident	Created incident	About to update severity	Updated severity	About to update milestone	Updated milestone

Webhook receiver output

% ruby print_http_body.rb 8300
[2021-06-24 13:50:23] INFO  WEBrick 1.6.1
[2021-06-24 13:50:23] INFO  ruby 2.7.2 (2020-10-01) [x86_64-darwin20]
[2021-06-24 13:50:23] INFO  WEBrick::HTTPServer#start: pid=22916 port=8300
{"object_kind":"issue","event_type":"issue","user":{"id":1,"name":"Administrator","username":"root","avatar_url":"https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon","email":"admin@example.com"},"project":{"id":21,"name":"autodevops","description":"","web_url":"http://192.168.0.202:3000/root/autodevops","avatar_url":null,"git_ssh_url":"ssh://git@192.168.0.202:2222/root/autodevops.git","git_http_url":"http://192.168.0.202:3000/root/autodevops.git","namespace":"Administrator","visibility_level":20,"path_with_namespace":"root/autodevops","default_branch":"master","ci_config_path":null,"homepage":"http://192.168.0.202:3000/root/autodevops","url":"ssh://git@192.168.0.202:2222/root/autodevops.git","ssh_url":"ssh://git@192.168.0.202:2222/root/autodevops.git","http_url":"http://192.168.0.202:3000/root/autodevops.git"},"object_attributes":{"author_id":1,"closed_at":null,"confidential":false,"created_at":"2021-06-24 17:51:40 UTC","description":"","discussion_locked":null,"due_date":null,"id":657,"iid":179,"last_edited_at":null,"last_edited_by_id":null,"milestone_id":59,"moved_to_id":null,"duplicated_to_id":null,"project_id":21,"relative_position":null,"state_id":1,"time_estimate":0,"title":"This is a new incident for which I will update the severity after creation","updated_at":"2021-06-24 17:51:40 UTC","updated_by_id":null,"weight":null,"url":"http://192.168.0.202:3000/root/autodevops/-/issues/179","total_time_spent":0,"time_change":0,"human_total_time_spent":null,"human_time_change":null,"human_time_estimate":null,"assignee_ids":[],"assignee_id":null,"labels":[{"id":109,"title":"incident","color":"#CC0033","project_id":21,"created_at":"2020-07-08 19:11:20 UTC","updated_at":"2020-07-08 19:11:20 UTC","template":false,"description":"Denotes a disruption to IT services and the associated issues require immediate attention","type":"ProjectLabel","group_id":null,"remove_on_close":false}],"state":"opened","severity":"unknown","action":"open"},"labels":[{"id":109,"title":"incident","color":"#CC0033","project_id":21,"created_at":"2020-07-08 19:11:20 UTC","updated_at":"2020-07-08 19:11:20 UTC","template":false,"description":"Denotes a disruption to IT services and the associated issues require immediate attention","type":"ProjectLabel","group_id":null,"remove_on_close":false}],"changes":{"author_id":{"previous":null,"current":1},"created_at":{"previous":null,"current":"2021-06-24 17:51:40 UTC"},"description":{"previous":null,"current":""},"id":{"previous":null,"current":657},"iid":{"previous":null,"current":179},"milestone_id":{"previous":null,"current":59},"project_id":{"previous":null,"current":21},"title":{"previous":null,"current":"This is a new incident for which I will update the severity after creation"},"updated_at":{"previous":null,"current":"2021-06-24 17:51:40 UTC"}},"repository":{"name":"autodevops","url":"ssh://git@192.168.0.202:2222/root/autodevops.git","description":"","homepage":"http://192.168.0.202:3000/root/autodevops"}}
::1 - - [24/Jun/2021:13:51:46 EDT] "POST / HTTP/1.1" 200 0
- -> /
{"object_kind":"issue","event_type":"issue","user":{"id":1,"name":"Administrator","username":"root","avatar_url":"https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon","email":"admin@example.com"},"project":{"id":21,"name":"autodevops","description":"","web_url":"http://192.168.0.202:3000/root/autodevops","avatar_url":null,"git_ssh_url":"ssh://git@192.168.0.202:2222/root/autodevops.git","git_http_url":"http://192.168.0.202:3000/root/autodevops.git","namespace":"Administrator","visibility_level":20,"path_with_namespace":"root/autodevops","default_branch":"master","ci_config_path":null,"homepage":"http://192.168.0.202:3000/root/autodevops","url":"ssh://git@192.168.0.202:2222/root/autodevops.git","ssh_url":"ssh://git@192.168.0.202:2222/root/autodevops.git","http_url":"http://192.168.0.202:3000/root/autodevops.git"},"object_attributes":{"author_id":1,"closed_at":null,"confidential":false,"created_at":"2021-06-24 17:51:40 UTC","description":"","discussion_locked":null,"due_date":null,"id":657,"iid":179,"last_edited_at":null,"last_edited_by_id":null,"milestone_id":59,"moved_to_id":null,"duplicated_to_id":null,"project_id":21,"relative_position":1879128549,"state_id":1,"time_estimate":0,"title":"This is a new incident for which I will update the severity after creation","updated_at":"2021-06-24 17:52:13 UTC","updated_by_id":1,"weight":null,"url":"http://192.168.0.202:3000/root/autodevops/-/issues/179","total_time_spent":0,"time_change":0,"human_total_time_spent":null,"human_time_change":null,"human_time_estimate":null,"assignee_ids":[],"assignee_id":null,"labels":[{"id":109,"title":"incident","color":"#CC0033","project_id":21,"created_at":"2020-07-08 19:11:20 UTC","updated_at":"2020-07-08 19:11:20 UTC","template":false,"description":"Denotes a disruption to IT services and the associated issues require immediate attention","type":"ProjectLabel","group_id":null,"remove_on_close":false}],"state":"opened","severity":"high","action":"update"},"labels":[{"id":109,"title":"incident","color":"#CC0033","project_id":21,"created_at":"2020-07-08 19:11:20 UTC","updated_at":"2020-07-08 19:11:20 UTC","template":false,"description":"Denotes a disruption to IT services and the associated issues require immediate attention","type":"ProjectLabel","group_id":null,"remove_on_close":false}],"changes":{"updated_at":{"previous":"2021-06-24 17:51:40 UTC","current":"2021-06-24 17:52:13 UTC"},"updated_by_id":{"previous":null,"current":1},"severity":{"previous":"unknown","current":"high"}},"repository":{"name":"autodevops","url":"ssh://git@192.168.0.202:2222/root/autodevops.git","description":"","homepage":"http://192.168.0.202:3000/root/autodevops"}}
::1 - - [24/Jun/2021:13:52:16 EDT] "POST / HTTP/1.1" 200 0
- -> /
{"object_kind":"issue","event_type":"issue","user":{"id":1,"name":"Administrator","username":"root","avatar_url":"https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon","email":"admin@example.com"},"project":{"id":21,"name":"autodevops","description":"","web_url":"http://192.168.0.202:3000/root/autodevops","avatar_url":null,"git_ssh_url":"ssh://git@192.168.0.202:2222/root/autodevops.git","git_http_url":"http://192.168.0.202:3000/root/autodevops.git","namespace":"Administrator","visibility_level":20,"path_with_namespace":"root/autodevops","default_branch":"master","ci_config_path":null,"homepage":"http://192.168.0.202:3000/root/autodevops","url":"ssh://git@192.168.0.202:2222/root/autodevops.git","ssh_url":"ssh://git@192.168.0.202:2222/root/autodevops.git","http_url":"http://192.168.0.202:3000/root/autodevops.git"},"object_attributes":{"author_id":1,"closed_at":null,"confidential":false,"created_at":"2021-06-24 17:51:40 UTC","description":"","discussion_locked":null,"due_date":null,"id":657,"iid":179,"last_edited_at":null,"last_edited_by_id":null,"milestone_id":null,"moved_to_id":null,"duplicated_to_id":null,"project_id":21,"relative_position":1879128549,"state_id":1,"time_estimate":0,"title":"This is a new incident for which I will update the severity after creation","updated_at":"2021-06-24 17:53:57 UTC","updated_by_id":1,"weight":null,"url":"http://192.168.0.202:3000/root/autodevops/-/issues/179","total_time_spent":0,"time_change":0,"human_total_time_spent":null,"human_time_change":null,"human_time_estimate":null,"assignee_ids":[],"assignee_id":null,"labels":[{"id":109,"title":"incident","color":"#CC0033","project_id":21,"created_at":"2020-07-08 19:11:20 UTC","updated_at":"2020-07-08 19:11:20 UTC","template":false,"description":"Denotes a disruption to IT services and the associated issues require immediate attention","type":"ProjectLabel","group_id":null,"remove_on_close":false}],"state":"opened","severity":"high","action":"update"},"labels":[{"id":109,"title":"incident","color":"#CC0033","project_id":21,"created_at":"2020-07-08 19:11:20 UTC","updated_at":"2020-07-08 19:11:20 UTC","template":false,"description":"Denotes a disruption to IT services and the associated issues require immediate attention","type":"ProjectLabel","group_id":null,"remove_on_close":false}],"changes":{"milestone_id":{"previous":59,"current":null},"updated_at":{"previous":"2021-06-24 17:52:13 UTC","current":"2021-06-24 17:53:57 UTC"}},"repository":{"name":"autodevops","url":"ssh://git@192.168.0.202:2222/root/autodevops.git","description":"","homepage":"http://192.168.0.202:3000/root/autodevops"}}
::1 - - [24/Jun/2021:13:54:02 EDT] "POST / HTTP/1.1" 200 0
- -> /

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