Add reCAPTCHA support to Review Board.

This change introduces optional support for captchas during the registration
process. reCAPTCHA is used for this, as it's pretty wide-spread, maintained,
has both visual and audio versions, works with or without JavaScript, and
has a nice side-effect of helping to scan old books.

Users can register accounts on http://www.recaptcha.net and enable captcha
support in the admin UI with the public and private keys provided on the
site.

This also splits off all the authentication-related settings into a dedicated
Authentication settings page.

Docs have been updated to talk about the new page and the captchas.

Tested the following scenarios:

* Registration without a captcha
* Registration with a captcha and a wrong answer
* Registration with a captcha and a right answer