[Bug 1045982] AoA bans for certain twitter handles

This adds a system for banning and unbanning certain twitter handles.
A user with the 'customercare.ban_account' permission may ban or unban twitter usernames from using the AoA tool at /army-of-awesome/moderate.

r?
pk needs to be set so the reply_to field in the request is correct (if we keep it hardcoded). Otherwise a DoesNotExist exception is thrown.