Refactor orderer to work on touch devices too

Summary: I ditched vmouse and implemented everything myself. I combined FloatingCard and DraggableCard into one class so that the DOM elements are reused when switching between the classes.

Test Plan:
With `make server PORT=9001`, go to

  http://127.0.0.1:9001/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20orderer%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22orderer%201%22%3A%7B%22type%22%3A%22orderer%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22options%22%3A%5B%7B%22content%22%3A%22%24x%24%22%7D%2C%7B%22content%22%3A%22%24y%24%22%7D%5D%2C%22correctOptions%22%3A%5B%7B%22content%22%3A%22%24x%24%22%7D%5D%2C%22otherOptions%22%3A%5B%7B%22content%22%3A%22%24y%24%22%7D%5D%2C%22height%22%3A%22normal%22%2C%22layout%22%3A%22horizontal%22%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D

on Chrome OS X and Safari iOS 7. It's reasonably snappy.

Reviewers: eater, brianmerlob

Reviewed By: brianmerlob

Differential Revision: http://phabricator.khanacademy.org/D8126