Adds read only and review mode to Perseus.

Summary:
Read only mode prevents answers from being changed. They can be set by
using the restore serialized state feature.

Review mode assigns classes (as in CSS classes) to each widget based on
whether each is correct or not. This allows special styling to me done.

Since these features are only needed by the SAT mission right now
they've only been implemented for radio widgets. Adding support for
additional widgets should be straightforward in the future though.

{F73452}

{F73453}

Test Plan:
Ran through an SAT section by first selecting answers then activating
review mode. Observed proper styling and read-only-ness.

Reviewers: aria, dylan

Reviewed By: dylan

Subscribers: kitt, #sat

Differential Revision: http://phabricator.khanacademy.org/D14275