Add in support for a new report a problem link

Summary: The actual DOM change will be done in a separate commit (on the main repo)

Test Plan:
loaded an exercise in tutorial mode
clicked on an adjacent video, clicked back
clicked on Report a problem, made sure form was thrown up

Reviewers: eater, alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D2557