Allow navigation in SAT mode.

Summary:
I didn't change khan-exercise.html with my changes to
javascript/exercises-package/problem-template.handlebars because it
wasn't necessary for the specific use case I'm looking at. I'd be
interested to know what I would have to do to khan-exercise.html
though.

This will be landed onto the SAT branch, to be used only by the SAT
branch in webapp.

Test Plan:
Tested with my current implementation of SAT mode and ensured that
forward and backward navigation work as expected.

Reviewers: dylan

Reviewed By: dylan

Subscribers: dylan, #sat

Differential Revision: http://phabricator.khanacademy.org/D13741