Add MWPS to custom notification settings
Release notes
You can now enable/disable email notifications for merge requests set to `Merge when pipeline succeeds' via a new setting in the custom notifications.
Problem to solve
Follow up from #211961 (comment 406172542)
As a user I want to control which email notifications I receive, namely, the new MR set to MWPS notification.
Intended users
User experience goal
Proposal
Users will be able to configure this notification:
Text should be: Merge when pipeline succeeds
