Add 'merge conflict event' notifications for custom notification levels
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=341102) </details> <!--IssueSummary end--> <!-- This template is a great use for issues that are feature::additions or technical tasks for larger issues.--> ### Proposal If a merge conflict happens in an MR, an email gets set to the assignee about this event. However, it is only sent if a user when a user has their [notification level](https://docs.gitlab.com/ee/user/profile/notifications.html#notification-levels) set to **Watch** or **Participate**. A 'merge conflict [event](https://docs.gitlab.com/ee/user/profile/notifications.html#notifications-on-issues-merge-requests-and-epics )' should be added so that users with a custom notification level ​may choose to receive this email with their custom settings. ### ### Workaround - Use **Watch** or **Participate** notification level.
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