Add 'merge conflict event' notifications for custom notification levels
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### 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.
issue