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

[Hocuri]

Fix #8195

The most interesting change is in imap.rs because there, messages are marked for immediate 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

[Hocuri]

This test tested how delete_server_after interacted with verified groups. We removed both features.

[iequidoo]

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?

Config::FixIsChatmail was added for this reason after CI switched to chatmail. But it seems that only test_dont_move_sync_msgs uses it to emulate a non-chatmail use case.

[Hocuri]

OK, new plan after talking to @link2xt:

We can likely do everything in delete_expired_imap_messages(). This function then has to be per-transport, and be called from imap loop.

If !context.get_config_bool(Config::BccSelf).await? && is_chatmail:

Just as today, filter by download state, downloaded messages are deleted immediately, and non-downloaded messages are deleted after some time. (but more than 2 days, which is the current time).

WRT Post-Messages:

• We have two imap entries, one for pre- and one for post-message (at least we should - I need to check)
• rfc724_mid on the msg contains the post message's Message-Id.
• pre_rfc724_mid contains the pre-message's Message-Id. We can select it, and delete it no matter the download state.