Send notification on group delete to all members

[PVince81]

Use background job because it could be a lot of users.

On the other hand, we need to find out the former members after the group was deleted.
Adding the list of members as a parameter to the background job is not an option because that array is likely to become big.

So it almost looks like we'd need a "deleted" flag first to the group is technically deleted and invisible, but the members data is still there until full deletion.

[PVince81]

Estimate: 3-5 md as it's quite complex and might require rewriting some part of the architecture to support a "delete" flag instead of deleting directly (includes time to make the decision)

[PVince81]

moving to triage for now