When a user deletes a message, we should rescind all the notifs about the original message. There are two issues:
- Multimedia messages are collapsed when they come from the same author - this diff disables this behavior
- We issue a special collapse key when a message creates a sidebar - this diff keeps this behavior, but as a result sidebar source notifications can't be rescinded
https://linear.app/comm/issue/ENG-10510/rescind-notifications-regarding-deleted-messages
Depends on D14561