Blocking assignee for merge requests

Merge requests will soon have multiple assignees. https://gitlab.com/gitlab-org/gitlab-ee/issues/2004

Some teams currently rely on the one-ness of the merge request assignee field to indicate that the person is blocking the merge request from moving on. (I'm using "blocking" here in a positive sense. There's nothing wrong with blocking a piece of work. It just indicates workflow.)

So the proposal here is that only one assignee can be the blocking assignee to retain that functionality.

This idea was floated in the most recent Create group conversation.