[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 have a `@` in the placeholder here, but if I type a `@` in the username, it won't work, right? You should either remove the `@` here or make sure that the backend will strip it out if it is there.