Add escalation policy dropdown to incidents sidebar

What does this MR do and why?

• Related issue: #330284 (closed)
• Related MR: !66165 (merged)

This MR adds an escalation policy dropdown to incident issues. This will be used in conjunction with a status field and escalation policies to let users to escalate incidents to their on-call teams.

Expected behavior:

• Tiering
  • GitLab Core: Escalation policy dropdown should not be available.
    • Placeholder for status dropdown should still be present. Escalation policy dropdown will never be available when status is not after !66165 (merged) merges.
  • GitLab Premium+: Escalation policy widget should be present in the sidebar.
• Permissions
  • Developer+ should be able to edit the escalation policy. Others should not see the Edit button.
  • Anyone who has permission to view the issue should be able to see the field in the sidebar
• Alert interactions
  • Incidents without an associated alert should show the Edit button
  • Incidents associated with an alert should not have an Edit button.
    • Existing incidents associated with an alert will show an empty link to the escalation policy.
    • New incidents created from alerts will show the link to the policy used for the alert.
• Help menu
  • If there are no escalation policies created in the project, the help menu should be available.
  • If there are escalation policies created, the escalation policy widget should be available.
  • Caveat: Only one escalation policy is currently allowed per project. The plan is to support multiple policies in future.
• Resolving/closing the alert/incident
  • Any escalation policy associated with the incident will stay associated with the incident, regardless of status/state changes

Screenshots or screen recordings

With escalation policies in project

Narrow sidebar - policy empty w/ hover	Narrow sidebar - policy populated w/ hover	Wide sidebar - policy empty (plus time tracking for regression check)	Listing policies in dropdown	Searching policies	Wide sidebar - policy populated	Hover state of policy w/ link	Selected policy in dropdown	With alert

Without escalation policies in project

Narrow sidebar - policy empty	Wide sidebar - help state collapsed	Help state expanded (plus time-tracking for consistency check)	Hover state of Learn More text

CE & alerts

Without alert	With alert	Logged out	Logged out with alert

Disabled states

Incident with disabled flag	Issue (with enabled flag)

Dark mode

Escalation policy help menu	Time tracking help menu

How to set up and validate locally

GitLab Core

1. Enable the feature flag

   Feature.enable(:incident_escalations)

2. Open a project, create an incident (/-/issues/new & select incident for issue type)
3. See the status placeholder and no escalation policy section or placeholder section

GitLab Premium - help menu

1. Navigate to Monitor > Escalation Policies & delete it if there's one there
2. Create an incident (/-/issues/new & select incident for issue type)
3. See the info icon & help menu in the Escalation policy widget

GitLab Premium - policy selection

1. Navigate to Monitor > Escalation Policies & create an escalation policy
2. Create an incident (/-/issues/new & select incident for issue type)
3. See the Escalation policy widget; policy selection, search, and deselection should all work
4. System notes corresponding to the triggered escalation policy should appear on the incident

With alerts

1. Navigate to Settings > Monitor > Alerts > Current integrations
2. Create & enable a new HTTP Endpoint alert integration if none exist
3. For an existing integration, click > Send test alert to trigger a test with a sample payload, like this one:
   • {"title":"Triggering a test alert to associate with an incident"}
4. Navigate to Monitor > Alerts to find & open the detail view of the new of alert
5. Click Create incident or View incident (whichever is fine; just depends on your incident-creation settings)
6. See escalation policy dropdown in the sidebar, with the Edit button absent

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.