this PR deletes all known messages belonging to a chat when the chat is
deleted.
this may not be an exhaustive list as a client might not know all
message-ids (eg. when using different times for "delete from device").
in this case, other devices may know more IDs. otherwise, the chatmail
server will eventually clean up at some point. for non-chatmail, this is
up to the user then.
the deletion sql commands were inspired by
[`delete_msgs_ex`](https://github.com/chatmail/core/blob/main/src/message.rs#L1743)
(in fact, [a first
try](https://github.com/chatmail/core/compare/r10s/clear-chat-on-delete)
was adapting that part, however, that seems less performant as lots of
sql commands are needed)
successor of #5007
bjoern
8e25639126