[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?
I think that makes sense for more complex data with more fields or longer strings, but IMO this is the easier route. I'll change it though if you'd like.