Created static rendering option for Perseus input-number and expression

Summary:
Mobile needs to be able to tell Perseus to render some input boxes as static TeX,
namely input-numbers and expressions. This commit adds an API option (staticRender) to enable
this behavior and renders appropriately in input-number and expression based on this option.
The static TeX still catches mouse events and calls onFocus. Fixes T2799.

Test Plan:
- Run Perseus.
- Enable `staticRender` in test.html.
- Create an input-number and expression, or use this [shortcut](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20input-number%201%5D%5D%5Cn%5Cn%5B%5B%E2%98%83%20expression%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22input-number%201%22%3A%7B%22type%22%3A%22input-number%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A%220%22%2C%22simplify%22%3A%22required%22%2C%22size%22%3A%22normal%22%2C%22inexact%22%3Afalse%2C%22maxError%22%3A0.1%2C%22answerType%22%3A%22number%22%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%22expression%201%22%3A%7B%22type%22%3A%22expression%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A%22%22%2C%22form%22%3Afalse%2C%22simplify%22%3Afalse%2C%22times%22%3Afalse%2C%22functions%22%3A%5B%22f%22%2C%22g%22%2C%22h%22%5D%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%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).
- Set their values using the console and verify that you get some nice, styled TeX.
- Enable the onFocusChanged handlers and verify that clicking between the TeX inputs triggers an event.

Reviewers: jack

Reviewed By: jack

Maniphest Tasks: T2799

Differential Revision: http://phabricator.khanacademy.org/D10742