Merge branch 'add_assign_to_me' into 'master'

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