Add reviewer filter to Vue merge request list app

What does this MR do and why?

For #442913 (closed)

Adds the "Reviewer" filter to the (feature-flagged) Vue implementation/refactor of the HAML MR list.

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
Filter option dropdown
Filtered by reviewer	x

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. Have or create an MR that has a reviewer
   • It will help to have at least one other MR that does not have a reviewer
3. Navigate to the Merge Request list e.g. http://gdk.test:3000/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing reviewer in the search bar and select the reviewer filter token
5. Select the user who is the reviewer for your MR in step 2
6. Submit the search / filter
   1. Try this with another user
   2. Try refreshing the page completely to test the URL search parameter on initial load
   3. Try clearing the reviewer token filter and submitting that to see all the MRs become visible
   4. Try using the "wildcard" options "Any" or "None" instead of a username

Caveats

Please note there is a known issue:

1. The current back end implementation does not support the "negated" (!=) filter, so that's simply not implemented here - which does not match with the non-Vue list