Send MDNs for messages deleted on the server

Now MarkseenMsgOnImap sends MDN even if it can't mark the message as
seen on the server.

To prevent multiple MDNs from being sent, MarkseenMsgOnImap is postponed
until the message is detected in a folder from which it is not going to
be moved.

python/tests/test_account.py

@@ -1542,7 +1542,6 @@ class TestOnlineAccount:
         assert msg.is_encrypted(), "Message is not encrypted"
         assert msg.chat == ac2.create_chat(ac4)
 
-    @pytest.mark.xfail
     def test_immediate_autodelete(self, acfactory, lp):
         ac1 = acfactory.get_online_configuring_account()
         ac2 = acfactory.get_online_configuring_account(mvbox=False, move=False, sentbox=False)