Merge approvals don't appear to trigger notification emails

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/9112 and https://gitlab.com/gitlab-org/gitlab-ee/issues/42
• Dev link: None
• GitLab version: Unknown

Expected behavior

Customer uses merge request approval flow. In cases where a merge request approver is specified the expectation is the user(s) receive a notification they have a merge request waiting.

Observed behavior

Merge request approvers do not receive notifications like assignees do.

Next steps

Estimate how much time this will take and which release we can get it in to.

cc/ @JobV