Threaded notifications in Slack
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### Description
We signed up our channel to get merge request events in Slack, but the channel gets updates for lots and lots of individual detail. It'd be easier to use if we could minimize the noise.
### Proposal
Have a single message, with threaded messages for updates. Keep the main message up to date with the status of the merge request (was it merged? closed? did it pass the pipeline? how many approvers are still needed?)
### Interested Customers
- [Ultimate, 220 seats](https://gitlab.my.salesforce.com/0016100001aJso6)
- [Ultimate, SaaS, 140 seats](https://gitlab.my.salesforce.com/0016100001beDS0)
- [Ultimate, 260 seats](https://gitlab.my.salesforce.com/0014M00001wUz3mQAC)
### Links / references
issue