Receive notification emails for merge request approval and approval removal

Description

• There should be notification emails for approving a merge request and removing the approval afterward, per the existing design of notifications.
• 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/views/notify/approved_merge_request_email.html.haml
  • https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/views/notify/approved_merge_request_email.text.haml

Text copy

Design

Approval

Unapporval