View system notes for merge request approval and approval removal

Description

• There should be system notes for approving a merge request and removing the approval afterward, per the existing design of system notes. In particular, this should be viewed in the merge request discussion thread.
• There is existing functionality here for the approval case. The scope of this issue is to cover the removal case, and also ensure that the text copy is as designed and consistent for the two cases.
• The existing functionality includes: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/services/system_note_service.rb#L465

Text copy

The text copy should be similar to this format:

• Victor Wu approved this merge request
• Victor Wu unapproved this merge request