Add telemetry for when the reviewer sidebar panel is opened

What does this MR do and why?

For #517652 (closed)

Adds a telemetry event to count when the Reviewers sidebar panel is opened.

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 instance that is either Premium or Ultimate (setup is outside the scope for these steps, you'll need a license key)
2. Create an MR.
3. Open the developer console Network tab
   • You may want to clear what's in there first to get a "fresh start"
4. From the MR "Overview" tab, open the "Reviewers" sidebar panel.
   • On larger screens, click "Assign" in the Reviewers section
   • On smaller screens, click "<< Expand", then "Assign"
5. Note the telemetry event is sent to /api/v4/usage_data/track_event