Added the Sorter widget, based on a reusable Sortable component.

Summary:
It dynamically resizes based on its contents, minimizing its footprint
as much as possible. This is by far the most compact draggy box widget.

Test Plan:
Created questions using the widget in test.html and on local webapp.
Tested the various options with plain text, TeX text, and images.

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6074