Update incident escalation policy if user sets alert incident status

What does this MR do and why?

Minor non-blocking UX improvement for &5716. (A little context: #345769 (comment 913312037))

When an incident was created prior to the enablement of the :incident_escalations feature flag, the incident has no corresponding IncidentManagement::IssuableEscalationStatus record. IE) The Status field on the incident says None. For an incident created from an AlertManagement::Alert, setting the status for the first time via the UI can also set the escalation policy for the incident - if the project has a policy under Monitor > Escalation Policies. This MR makes it so the incident sidebar updates the policy field without a page refresh in that scenario.

Screenshots or screen recordings

How to set up and validate locally

1. Create an escalation policy
   1. Nav to Monitor > Escalation Policies
   2. Create a new one & route on-call pages directly to a user (so you can skip setup for an on-call schedule)
2. Create an alert
   1. In a project with maintainer+ permissions, nav to Settings > Monitor > Alerts
   2. Create a new alert integration (name it, set it to active, skip everything else, click save)
   3. Create a new test alert with a payload like { "title": "This is my test alert" }
3. Create an incident from the alert (Create incident button on alert detail page from Monitor > Alerts)
   • Option 1: Create incident w/ feature flag disabled OR

     Feature.disable(:incident_escalations)

   • Option 2: Delete escalation status record in rails console after incident creation

     Issue.incident.last.escalation_status.destroy!

4. Open the incident
5. Set the Status field to any value other than None
6. The Escalation policy field should have a value w/o refreshing the page

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.