Let users know why they can no longer approve MR after rebasing

Problem

Users who were previously eligible to approve an MR are no longer eligible after adding commits. This may add confusion when reviewers rebase a branch (not adding any additional changes) and find themselves no longer eligible to approve.

Proposal

Add some wording letting users know why they are no longer eligible to approve. (see designs)

Testing

Update (or add) spec/features test to ensure message is displayed when user cannot approve after a rebase. SET: @jay_mccure