Delay load video resources for related videos.

Summary:
A lot of exercises don't have a related video, and even those that do
can afford to wait a little longer to render the related video
thumbnail; those resources shouldn't block rendering of the exercise
itself.

Test Plan:
python -m SimpleHTTPServer, went to /exercises/subtracting_decimals.html made sure it loaded fine

on real KA dev app server, made sure I was in the a/b test where the related videos should be shown, then went to subtracting_decimals, made sure video thumbnail worked, clicked on it

Reviewers: alpert, cbhl

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2141