feat: protect the Date header

[link2xt]

Closes #6878
Based on #6867
This depends an a fix #6669, so we may want to postpone merging this until more users have it.

We do not try to delete resent messages
anymore. Previously resent messages
were distinguised by having duplicate Message-ID,
but future Date, but now we need to download
the message before we even see the Date.
We now move the message to the destination folder
but do not fetch it.

It may not be a good idea to delete
the duplicate in multi-device setups anyway,
because the device which has a message
may delete the duplicate of a message
the other device missed.

To avoid triggering IMAP move loop
described in the comments
we now only move the messages
from INBOX and Spam folders.

[link2xt]

Legacy Python test tests/test_1_online.py::test_resend_message got broken by this.
Resent message is not recognized as duplicate anymore because outer timestamp of just received message is compared to the timestamp of the fetched message:

core/src/imap.rs

Lines 603 to 610 in adcc8a9

	let ts_sent = headers
	.get_header_value(HeaderDef::Date)
	.and_then(|v| mailparse::dateparse(&v).ok())
	.unwrap_or_default();
	let is_dup = is_dup_msg(is_chat_msg, ts_sent, ts_sent_old);
	if is_dup {
	info!(context, "Deleting duplicate message {message_id}.");
	}

[link2xt]

This has now broken the test added in c14f45a

[link2xt]

We will have to use something less suspicious than unix epoch, GMX is unhappy:
#6878 (comment)