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

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>