Assign user when MR builds pass

Description

I often find myself waking up to MRs that have passed builds but have not yet been passed to a reviewer. This is because of last minute fixes being pushed just before finishing work/the day. With remote companies, the time any one person is asleep is often time someone else is awake, to some extent. There is also more generic scenarios where a break from the GitLab interface will occur after some final changes are pushed, where the author will return to passed builds on their MR but they have not assigned to the reviewer yet.

Proposal

I propose that we have an MR assignment feature similar to "Merge when builds pass"; "Assign when builds pass".

My proposal for the first iteration is that a user can be selected to be assigned when the MRs builds pass.

This could be related to gitlab-ce#22043, allowing GitLab to automatically assign the best reviewer at the time of builds passing.