[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 use [`get_or_create`](https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create).