Trigger merge status subscription when a new discussion is created

What does this MR do and why?

When a new unresolved discussion is created (e.g. via a review or starting a new thread), it affects the mergeability of the merge request.

Since we need to update the merge widget in real-time, we need to let the subscribers know about it. To do this, we trigger the subscription whenever a new unresolved discussion is created.

How to set up and validate locally

1. Enabled the realtime_mr_status_change feature flag so the frontend can update real-time.
2. Open 2 browser windows viewing the same MR.
3. Watch window B.
4. On window A, start a new thread, comment on a diff line or submit a review. The merge widget on Window B should update in real time.

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.

Related to #372643 (closed)