Remove underlines in related video links

Summary: Side effect of base styles changing to default text-decoration: underline, now we have to try if we don't want the underline

Test Plan:
Before: looked like the top link
After: looks like the bottom link
https://s3.amazonaws.com/uploads.hipchat.com/6574/17223/W7uzKZ3CN1htQXm/upload.png

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D5022