DONE Add CAPTCHA to registration

[VinnyVideo]

I've never been a fan of the "select pictures of cars" captcha and its brethren. A lot of the pictures are blurry or ambiguous, and it almost always takes me several attempts to get it right. This is a suboptimal user experience, and there's a risk it could cause legitimate users to become frustrated and give up on registering.

(Fortunately, it seems that we're using a different captcha right now.)

Course clear! You got a card.