Add "Reviewers" to Vue MR list page

What does this MR do and why?

For #478232 (closed)

Shows the list of reviewers for a Merge Request on the Vue MR list page.

Note: This requires the vue_merge_request_list feature flag to be enabled.

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

Before	After

How to set up and validate locally

1. Enable the vue_merge_request_list feature flag in the rails console rails c => Feature.enable( :vue_merge_request_list )
2. Add a reviewer (or 5) to an MR
3. Go to the Merge Request list page e.g. http://gdk.test:3000/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Note the "Reviewers" area of the MR you added reviewers to
   1. Bonus: Switch to the master branch and note that it disappears, go back to this branch, etc. etc.