Add slash command to promote issue to incident

What does this MR do and why?

Implements #296787 (closed)

Adds /promote_to_incident quick command to promote issue to incident.

Screenshots or screen recordings

promote_to_incident_DEMO

How to set up and validate locally

1. Create a new issue
2. Create a comment with /promote_to_incident command
3. Verify the issue converted to an incident

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.