Add a "Status" dropdown in the UI on the incident page

Motivation

When a user is notified of an incident via an escalation policy, the user needs a way to acknowledge or resolve the page/incident. This issue represents the work needed to allow users to view & update the "status" of an incident. This work can be done independently of escalation policies themselves.

Scope

• Show the current status of the manually created incident
  • The options should match the alert status dropdown
• Show the current status of the alert associated if the incident is from an alert
• Allow the user to update the status of the incident using the dropdown
  • Edit button should only be available for developer+
  • If the incident was manually created, this should update the associated IssuableEscalationStatus#status
  • If the incident was created from an alert, this should update the alert's status
• This includes both the frontend and backend API to support the UI.

Out of scope: Handling changes to escalations based on status

UX

Status and escalation policy sidebar items	Status dropdown options	More information about what changing the status does

Proposed behavior

👇 See #330283 (comment 736328526) for discussion.

Scenario	License	Has Alert	Has Escalation Policy (One)	Has Escalation Policy (Many- Future State)	Result / Behavior
A	Core				Show status field, status is set manually by the user and is not tied to an Alert or Escalation Policy.
B	Core	Y			Show alert status, alert status is updated if incident status changes
C	Premium				Show status field, status is set manually by the user and is not tied to an Alert or Escalation Policy.
D	Premium	Y			Show alert status, alert status is updated if incident status changes
E	Premium		Y		Show Escalation Status, Default is None. Discussion ,this might require a change to the data model b/c there isn’t the concept of None yet.
F	Premium			Y	Show Escalation Status, Default is None.
G	Premium	Y	Y		Show Escalation Status as Read only, Default is Alert Status. Escalation policy has already been triggered from the alert. Alert status is updated if incident status changes
H	Premium	Y		Y	Future state for when there are multiple Escalation Policies.