Send an email notification when "merge when pipeline succeeds" is set on a merge request

Problem

We get notified by email when an MR is approved, but we don't know when the reviewer sets MWPS. Usually, we'd have to check the MR manually or wait to receive a notification when it is merged.

It is useful to tick off the MR mentally when MWPS is set.

Proposal

An email will be sent to notify the subscribers in the MR thread, when MWPS is set.

The text should be: Your merge request has been set to merge when the pipeline succeeds

Users will be able to configure this notification:

Text should be: Merge when pipeline succeeds