[Design] Incorporate approval rules when assigning reviewers to a merge request

Proposal

Currently the approver's widget shows you who should be assigned as a reviewer on a merge request by listing out approval rules and the people who satisfy them. However, you can't actually assign reviewers from the approver's widget. This issue proposes that we give users a new way to assign reviewers that incorporates approval and code owner rules.

The goal is to help an MR author know who to assign, and to confidently assign the correct people as reviewers on a merge request.

Edited by Matthew Nearents