Add telemetry for when a user requests a review using the traditional reviewer dropdown

What does this MR do and why?

For #517652 (closed)

Adds a tracking event - user_requests_review_from_mr_simple_sidebar - to the "edit" button in the Reviewers section of the MR sidebar.

Screenshots or screen recordings

N/A, all backstage

How to set up and validate locally

1. Have an MR in a project with at least one assignable user (probably two - including yourself - would be better)
   • You must be able to "admin" the MR, or basically "edit it, including add reviewers"
2. Visit the MR and stay on / confirm you are on the "Overview" tab
3. Click the "Edit" button/link in the sidebar in the "Reviewers" section
4. Add a new reviewer using the dropdown
   • Make sure you "finalize" the change by clicking away from the dropdown to close it
5. Note the request sent to /api/v4/usage_data/track_event with the event user_requests_review_from_mr_simple_sidebar

MR acceptance checklist

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