Show related videos in mastery tasks.

Summary:
The current feedback is that videos can be helpful in mastery tasks. Long
term we want to A/B test the effects of this, but for now, just show them
unless we're close to mastery.

Test Plan: do a couple of mastery tasks, make sure nothing blows up.

Reviewers: alex, zach

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4605