Filter current user to top of assignee list in issues/merge requests ### What does this MR do? Fixes #2076 Filter the current user to the top of the assignee list in issues and merge requests. This makes it fast/easy for a user to assign themselves without compromising the UI. This is the same thing we do in the filter select boxes in other places. ### Are there points in the code the reviewer needs to double check? No. ### Why was this MR needed? Users requested the feature in #2076 See merge request !1493