IMAP: ignore unsolicited LIST and STATUS before we're authenticated

Continuing from !5 (closed)

These responses were combined with legit responses which arrive from the real server later on, after having authenticated, and as a result, a malicious active attacker on a network was able to inject extra fake mailboxes to the GUI even when upgrading connection from plaintext to a secure one via STARTTLS.

These two responses are the only ones which are handled by our pre-task code directly in Model.cpp, which means that the state machine has to be checked and messages which are received prior to authentication must be rejected. Since this is really a pretty serious violation of the IMAP state machine, let's not ignore them silently, and simply treat them as any other unexpected response.

BUG: 432353
Change-Id: I9292fcb20215ebe4dbc7a103fc9403dfa97b258b
Reported-by: Damian Poddebniak
Co-authored-by: Espen Sandøy Hustad <espen@ehustad.com>
Reviewed-by: Caspar Schutijser <caspar@schutijser.com>