Request to allow Merge Request UI to filter by user merge_user or Merged_By

Problem to solve

When viewing a list of merge requests it can be important to see who merged those items. Currently it's not possible to filter the list of merge requests to merge_user.

Proposal

Add a new filter to the merge request dashboard that allows filtering of merge_user. The merge_user field is available via REST/GraphQl, so we should be able to query it against the database as well for the dashboard.