Expression input 2.0: MathQuill

Summary:
This commit replaces our previous "enter math as plain text, get LaTeX render"
with the [MathQuill dev branch](https://github.com/khan/mathquill/tree/dev).

- `MathInput` encapsulates `MathQuill.MathField` in a React component
- `InputWithExamples` can now toggle between basic text input and `MathInput`
- `KAS` is now included locally in `lib/` for easier development

Test Plan:
Tested rendering, answering, and general interaction in test.html on desktop
Chrome, Firefox, and Safari: everything seems to work! Also tested on iOS and
Android and got various bugs.

Reviewers: jack

Reviewed By: jack

Subscribers: elizabethlin, khirasaki, eater

Differential Revision: http://phabricator.khanacademy.org/D9014