Only show clues to people in the appropriate experimental group and mark conversions.

Summary: Only show clues to people in the appropriate experimental group and mark conversions. See @D11772 and @D11733.

Test Plan: See @D11772 and @D11733

Reviewers: aria, eater

Reviewed By: eater

Subscribers: jace, mattfaus, lauren, tony, petergao, eliana, tom, alan, marcia

Differential Revision: http://phabricator.khanacademy.org/D11988