Add ability to subscribe/unsubscribe to Slack notifications about an MR
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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.
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- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=344030)
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See [parent epic](https://gitlab.com/groups/gitlab-org/-/epics/6981) for more context.
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