Username blacklist

Takes @BigBlueHat's username blacklist and adds a validator to the registration form that prevents users registering with usernames from the blacklist.

Obviously this is my first attempt to commit to h, so it's very likely I've broken the world, violated coding standards, and generally made a mess of it. Let me know if I did!
Works as is, but should this be `__package__` instead of `__name__`?