Add A/B test for clues in radio

Test Plan:
1) In order to run, you'd `make all` in perseus (assuming you have an existing webapp)
2) Should show for half the users, and not show for the other half

Reviewers: eater

Reviewed By: eater

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7369