Added a shuffling option that ensures a distinct permutation.

Summary:
If such a permutation is possible, of course. Also threaded the seed used by
the shuffle method (the current problem number) to all widgets, and added a way
to see it in action on test.html

Test Plan:
Tested that each time I reload test.html, the permutation of the Sorter and
Matcher widget options changes.

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6039