feat: Remove "Delete Messages from Server" (delete_server_after) config option, second try

[Hocuri]

test_delete_expired_imap_messages() is AI-generated and needs to be reworked. Everything else is written by myself and ready for review as soon as the tests pass.

Supersedes #8217, fix #8195. If you want to see the difference, call git diff hoc/remove-delete_server_after hoc/remove-delete_server_after-2 (only works until someone rebases again, of course).

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

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 AFAIK?

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 fetch_many_msgs() whether the server is Nauta, probably by querying the transports SQL table

TODO:

• test_delete_expired_imap_messages() is AI-generated and needs to be reworked
• Find out why test_imap_autodelete_fully_downloaded_msg fails now (it passed in feat: Remove "Delete Messages from Server" (delete_server_after) config option #8217 IIRC)
• Remove all the debug statement and the TODO (grep for "dbg" and "TODO")