fixed touch-related bugs on the orderer widget

Summary:
Previously, (1) orderer broke on mobile if any given card was rapidly pressed.
Separately, (2) the orderer would break if you clicked on a card while a different card
was animating back into position. Now, the orderer correctly unbinds Util.touchHandlers
when it's unmounted, and refuses to accept clicks on cards when a separate card is animating.

Fixes T2576
Fixes T2588

Test Plan:
- Run Perseus on a touch device.
- Create an Orderer ([shortcut](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5Cn%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%2C%7B%22content%22%3A%22%24z%24%22%7D%5D%2C%22correctOptions%22%3A%5B%7B%22content%22%3A%22%24x%24%22%7D%2C%7B%22content%22%3A%22%24z%24%22%7D%5D%2C%22otherOptions%22%3A%5B%7B%22content%22%3A%22%24y%24%22%7D%2C%7B%22content%22%3A%22%24z%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))
- Drag down a few cards.
- To test (1): Click the same card as quickly as possible several times in a row; notice that the widget does not break!
- To test (2): Drag one of the cards far away from its final position and click a different card as it animates back into place; notice that the widget does not break!

Reviewers: jack

Reviewed By: jack

Subscribers: eater, alpert

Maniphest Tasks: T2588, T2576

Differential Revision: http://phabricator.khanacademy.org/D9502