Add a "historical" suggested position to the telemetry reported when selecting a reviewer

What does this MR do and why?

For #519368 (closed)

As the title says, this adds a "previous position" value to the reported telemetry when a reviewer is selected.

This might be a slightly complex concept, so please review the issue linked above for a more thorough description of what this is doing.

Screenshots or screen recordings

N/A, this is all backstage

How to set up and validate locally

1. Have a Premium or Ultimate GDK
2. Go to an MR in a project with at least 2 users (3, if you want to exclude yourself)
3. In the MR sidebar Reviewers area, click "Assign"
   • 
4. Add a reviewer in the "Assign reviewers" sidebar
   • 
5. Blur / Close the reviewers dropdown
6. In your browser console's Network tab, confirm that the correct track_event network request is sent.
   • The events are user_selects_reviewer_from_mr_sidebar and user_selects_reviewer_from_mr_sidebar_after_search (if a search term was in the dropdown when you selected a reviewer)
   • The payload should match { ... "additional_properties": { "value": #, "suggested_position": $ } ... }, where # is the position of the reviewer you selected beginning after any other already-selected reviewers and $ is where that selected user appeared in the unfiltered/"no-search" list beginning after any other already-selected reviewers.

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