Add telemetry when the user self-assigns as a reviewer

What does this MR do and why?

For #517652 (closed)

Adds a tracking event - assign_self_as_reviewer_in_mr - to the "assign yourself" button in the Reviewers section of the MR sidebar.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

All backstage

How to set up and validate locally

1. Have an MR with no reviewers assigned
   • You must be able to "admin" the MR, or basically "edit it, including add reviewers"
2. Click the "assign yourself" button/link
3. Note the request sent to /api/v4/usage_data/track_event with the event assign_self_as_reviewer_in_mr