Custom notification for changing milestones

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Follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/51520 to bring this notification event to the customized notification level functionality. For both Web UI and and API. I.e. it would add an option to:

Screen_Shot_2018-09-20_at_14.43.00

Edited by 🤖 GitLab Bot 🤖