Adding custom notifications for added approver

What does this MR do and why?

In 16.7 we added Added approver notification setting and #12855 (closed).

We had a link to our documentation on the release post here but when I followed this through to the docs page I couldn't see any reference to this notification type.

This is a feature that was highly requested by our customers and I think they're currently working it out from landing on this comment at Optional Merge Request notifications for approvers.

I'm not sure if this should also be listed under user notification events here because this is a user event as well as a merge request event however I think having it under the merge request events is a good start.

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