Filter merge requests by individual approvers

Description

• Filter merge requests on the merge requests list view (e.g. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests) to find merge requests that have specified individual approvers.
• If the search has approver: @victorwu, it would find all merge requests that have @victorwu explicitly added as an individual approver.
• If the search has approver: @victorwu approver: @smcgivern, it would find all merge requests that have both @victorwu and @smcgivern explicitly added as an individual approver.
• Suppose @victorwu is part of Group A. A given merge request has Group A (and only Group A) as an approver (group approver). In this case, if the search is approver: @victorwu, it would not return this particular merge request. This case is not covered in this issue.

Design

Design	Notes
	Use approval icon (link) for the approver dropdown option. Fill color: #2E2E2E (same as the other icons)
	The first option should be None - Matches MRs without any approvers

Original description

It would be very useful to be able to filter merge requests based on the approvers included in the merge request. This way users can easily find the merge requests in which they are involved as approvers, and require their attention.

Proposal

A filter similar to the Authors/Assignee/Milstone/Labels filters in the Merge Requests tab.