[UX] Display reason why user can't approve a MR

Proposal

I just spent a couple hours trying to figure out why a custom merge request approval rule wasn't working. I eventually realized that I had a commit that was part of the MR, which disqualified me from approving.

It would be really great if on the approvers section of an MR, where it shows all the user icons, if it also included the users who were disqualified. Obviously they should be greyed or x-ed out or something, with a hover-over pop-up tip explaining why that user can't approve this MR. (i.e. "$User can't approve this PR since they have a commit in this PR. You can change these settings here (Link to the MR approval settings).