Do not erase contact when banned

Stop erasing banned accounts / removing from the database when
contactRemoved signal is received from the daemon. Instead mark the
account as banned and force the client to refresh its filters.

Also: Fix the default account filter so it filters out banned accounts
unless regex is a perfect match.

Change-Id: I75d1aea5271d88b13f1850d47b33c279b89a5a05
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>