Feature Request: Add notification for own merge

Proposal

A GitLab Ultimate customer (ZD ticket, internal only) recently inquired about how to get notifications for self-merging. That's to say, they would like a notification to be scheduled to go out any time a developer merges their own commit.

It's true that you can already prevent developers from merging their own commits, though that's not what they're trying to do. In case of emergency, sometimes one of their developers must merge their own code. As such, when this occurs, the customer would very much like for there to be an email notification, which would then kick-start a post-merge review.

To quote from the customer:

We need to have a solid net for catching this and not be depended on that someone in a crisis mode where a fix is urgent needs to remember to add someone to the MR to ensure notification. Then they could just as well just send the mail them self. Also we would like this form preventing unwanted situations where someone by accident or by own will merges own code. This is something we would have to write a script for using API and check mergers and contributors, but that would be clunky.