Add the ability to block single ip addresses

Summary: Include the person who was trying to SQL inject us, by default.

Test Plan:
Go to the site, ensure that it loads normally. Add yourself to the
BLOCKED_IPS list, check that it returns 403 forbidden.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2053