Add Escalation policy dropdown in the UI

Motivation

Users need an ability to trigger notifications & escalations for a manually created incident. Unlike alerts, incidents will not default to paging immediately, as users may spend additional time adding context after creating an issue before paging should start.

Scope

• Show users which Escalation Policy is currently being used for the associated alert, if there is an alert
• Show users which Escalation Policy is currently being used for the manually created incident, if any
• Show 'Learn More' section if no policies are set up for the project
• Allow users to start using a particular Escalation Policy for the incident
  • Edit button should only be available for developer+
  • Edit button should not be available on incidents which have an associated alert
  • The Escalation Policies available in the dropdown should include all policies in the project
  • This should create a IncidentManagement::IssuableEscalation to begin escalations
  • Any EscalationRules which are set to occur after 0 minutes should be executed immediately
• Allow users to switch which Escalation Policy is being used for the incident
  • Switching the Escalation Policy should switch the status of the incident to Triggered
• This issue covers both the frontend changes and the backend API requirements.

UX

Escalation policy dropdown	No escalation policies created	No escalation policies created - expanded	Hidden edit button for those without proper permissions