Add filter for reviewer to MR list filter bar

Problem to solve

Once #216054 (closed) is completed, users will be able to designate a reviewer for a merge request. The assigned reviewer however will not have a mechanism to find which MRs they have been designated as a reviewer.

Proposal

Introduce a new filter option Reviewer that returns a list of MRs in which a user has been assigned as the reviewer.

• Filter by a given reviewer_username param
• Filter by a given reviewer_id param (seems to be used mainly for API)
• Filter by NO reviewer
• Filter by ANY reviewer
• Negated filter by a given reviewer_username param