Add escalation status for incident issues

What does this MR do?

Related issue: #330277 (closed)

• Adds an IssuableEscalationStatus model for issues.
  • This will allow incident issues to be marked as triggered/acknowledged/resolved. (GitLab Core)
  • Ultimately, the status will be used to indicate whether paging should occur for the issue (GitLab Premium ) [comparable to alerts]
• Refactor status-related logic from AlertManagement::Alert & specs into an Escalatable module which both alerts & the IssuableEscalationStatus model

Migration Info

Up output

% bin/rails db:migrate
== 20210729202143 CreateIncidentManagementIssuableEscalationStatuses: migrating 
-- create_table(:incident_management_issuable_escalation_statuses)
   -> 0.0457s
== 20210729202143 CreateIncidentManagementIssuableEscalationStatuses: migrated (0.0458s) 

% gdk psql
psql (12.7)
Type "help" for help.

gitlabhq_development=# \d incident_management_issuable_escalation_statuses
                                             Table "public.incident_management_issuable_escalation_statuses"
         Column         |           Type           | Collation | Nullable |                                   Default                                    
------------------------+--------------------------+-----------+----------+------------------------------------------------------------------------------
 id                     | bigint                   |           | not null | nextval('incident_management_issuable_escalation_statuses_id_seq'::regclass)
 created_at             | timestamp with time zone |           | not null | 
 updated_at             | timestamp with time zone |           | not null | 
 issue_id               | bigint                   |           | not null | 
 policy_id              | bigint                   |           |          | 
 escalations_started_at | timestamp with time zone |           |          | 
 resolved_at            | timestamp with time zone |           |          | 
 status                 | smallint                 |           | not null | 0
Indexes:
    "incident_management_issuable_escalation_statuses_pkey" PRIMARY KEY, btree (id)
    "index_uniq_im_issuable_escalation_statuses_on_issue_id" UNIQUE, btree (issue_id)
    "index_im_issuable_escalation_statuses_on_policy_id" btree (policy_id)
Foreign-key constraints:
    "fk_rails_29abffe3b9" FOREIGN KEY (policy_id) REFERENCES incident_management_escalation_policies(id) ON DELETE SET NULL
    "fk_rails_f4c811fd28" FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE

Down output

% bin/rails db:migrate:down VERSION=20210729202143
== 20210729202143 CreateIncidentManagementIssuableEscalationStatuses: reverting 
-- drop_table(:incident_management_issuable_escalation_statuses)
   -> 0.0275s
== 20210729202143 CreateIncidentManagementIssuableEscalationStatuses: reverted (0.0354s) 

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #330277 (closed)