[merge request approvals] It's unclear why approvals are rescinded when a new commit is added

Relevant slack thread

Problem

An approval can be automatically removed if a commit is added to an MR that updates one of the Codeowner approved files. There is a system note that says @user approved but no indication of the removal.

Proposal

Make it clear why the approval was removed by adding a system note to the activity:

@author revoked approvals from @reviewer1, @reviewer2 and @reviewer3 by pushing to the branch

Based on discussion in #382087 (comment 1579491095), we're planning to:

• Utilize event store when removing approvals
• Create a simple system note when approvals are reset