Limit availability of page quick action to relevant incidents

What does this MR do and why?

This MR fixes two issues with the /page quick action, which escalates incidents to on-call users.

1. The /page quick action should only be available to GitLab Premium+.
2. The /page quick action not be available for incidents which have alerts. Alerts escalate automatically, so the associated incidents should not.

Screenshots

Incident without alert	Incident with alert

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:incident_escalations)

2. Create an incident in an unlicensed project on EE (new issue with type of incident)
3. Type /page in a comment field to see that the quick action option is not available
4. Create an incident from an alert
   1. Go to Settings > Monitor > Alerts
   2. Create an alert integration & send a test alert (sample payload: {"title": "New alert"})
   3. Go to Monitor > Alerts & open the alert detail
   4. Click "Create incident"
5. Type /page in a comment field to see that the quick action option is not available

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.