Filter merge requests by approver

Open username-removed-647088 requested to merge heijmans/gitlab-ee:52-filter-by-approver into master

What does this MR do?

Add a filter to the merge request lists to enable the user to filter by approver.

Why was this MR needed?

When there are many merge requests, it is very inconvenient to browse through all the merge requests to view the approvers. A common use case is to view all the merge requests the current user needs to review/approve.

What are the relevant issue numbers?