fix: Correctly delete unneeded messages on the server; Remove "Delete Messages from Server" (delete_server_after) config option

[Hocuri]

Supersedes #8217, fix #8195.

The most interesting change is in delete_expired_imap_messages() because there, messages are marked for deletion on single-device chatmail profiles.

The logic in delete_expired_imap_messages() was wrong before, and pre-messages were not deleted at all. This is fixed by also querying pre_rfc724_mid in addition to rfc724_mid from the msgs table. In order not to make the SQL statement too complex, I split it into two.

WRT tests:

• test_immediate_autodelete() tests the auto-deletion; test_imap_autodelete_fully_downloaded_msg() tests that even for a message that is split into pre- and post-message, both messages are deleted.
• A lot of tests test that if bcc_self is on, messages are not auto-deleted. E.g. test_one_account_send_bcc_setting() and test_markseen_message_and_mdn() relies on the fact that messages stay on the server when bcc_self is on.
• Unfortunately, it is not possible to test that messages are only deleted for chatmail servers, because we don't have any tests that use a non-chatmail server.

Open question: Do we want to treat Nauta.cu like a chatmail server, in order to keep the old behavior on Nauta of auto-deleting? If not, I'll tell @adbenitez how he can do it in AC. In order to do this, need to find out in fn should_delete_all_downloaded_messages() whether the server is Nauta, probably by passing the transport_id and querying the transports SQL table.

[iequidoo]

Looks good, but not sure this should be merged right now because of the discussion at #8195 (comment)