checkboxes no longer show up next to unchecked checkboxes, only appear when checked

Summary: instead of always setting the content on the checkbox, only apply it when ::checked. it's a one line diff, so apologies for how heavyweight this is, but i don't know what the norms are around perseus land for landing diffs like this

Test Plan: see http://dl.dropboxusercontent.com/u/406291/Screenshots/3e9a.png

Reviewers: eater, jack

Reviewed By: jack

Subscribers: charlie, jack

Differential Revision: http://phabricator.khanacademy.org/D11685