Destroy conversation when all participants leave it

When all participants of a conversation leave it, conversation remains in db.

Due to private issues, when nobody is seeing it, it should be destroyed.

Test is long but I think it accurately ensures expected behavior.
This part should be replaced by specifically crafting a conversation that only has one participant in the first place as setup for the next part. Then this should be a second test whether the new code part is triggered if there still is a participant left.