Make a draggy card orderer

Summary:
Add a widget that allows users to pull cards from a bank of cards and
order them to make an answer.

Test Plan:
Make an exercise, add the ordering widget. Add some cards to the
correct answer, and some other cards to the other cards. Make sure that the
numbers, symbols, and variables are sorted into different sections. Drag some
cards into the box. Make sure that they go in the right places. Make sure that
they animate going to the right places. Remove some cards from the box, try
putting them back into the box, and above back into the bank. Make sure they
disappear when they go into the bank. Make the correct answer out of the cards.
Make sure it is scored as correct.

Reviewers: jakesandlund, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2821