feat: delete messages on IMAP when deleting chat

[r10s]

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 (in fact, a first try was adapting that part, however, that seems less performant as lots of sql commands are needed)

successor of #5007

[iequidoo]

Looks good, but if bcc_self is disabled, sync messages are not sent, so the result on other devices depends on whether they are online and have already received messages. I think that it's supposed that other devices are online and should still have messages deleted on the current device in this case

[link2xt]

bcc_self means "multi-device", so if you have a multi-device setup with bcc_self set to false, you have a broken multi-device setup.

[link2xt]

I thought this might have weird consequences in large groups. If the message is split into multiple SMTP messages, then second device might receive a message and then delete the chat. Then first device will remove the message from SMTP queue and stop sending the rest of the copies.

But all is fine: we always place bcc-self copy as the last copy into smtp queue.

[iequidoo]

bcc_self means "multi-device", so if you have a multi-device setup with bcc_self set to false, you have a broken multi-device setup.

Generally, yes, but sometimes i disable bcc_self to make some actions that shouldn't be synced to other devices, e.g. i have different muted chats on devices because i use them for different purposes. Not that important though, and probably nobody deletes messages this way.