[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?
You should make and use a modelmaker, [like these](https://github.com/mythmon/kitsune/blob/master/kitsune/wiki/tests/__init__.py#L19).