Status messages: Sync with Slack
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### Problem to solve
We'll soon have user status messages in GitLab via https://gitlab.com/gitlab-org/gitlab-ce/issues/35463, including API endpoints.
Let's explore if we can find a way to ship an integration that allows syncing of this status with Slack, as this idea was inspired by their cool feature.
### Proposal
If a Slack integration is set up, Slack status messages should sync with GitLab status messages in both directions.
* When I add/delete a status in Slack, it is synced to GitLab.
* When I change/overwrite/delete the status in GitLab, it is synced back to Slack as well.
* This integration detail can be enabled/disabled on Admin-level.
### What does success look like, and how can we measure that?
(If no way to measure success, link to an issue that will implement a way to measure this)
issue