No longer auto-capitalize or auto-correct for math inputs.

Summary:
Previously, some inputs that actually handled math were auto-capitalizing and auto-
correcting, such that if you typed a lower-case variable name, it would be capitalized and
marked as incorrect, which is a huge pain on touch devices. Now, those inputs are defaulted
with this behavior turned off.

Test Plan:
- Turn off useMathQuill in test.html.
- Turn on your favourite touch device.
- Create an expression widget ([shortcut](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5Cn%5B%5B%E2%98%83%20expression%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%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%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)).
- Type in a single-word variable name, such as 'p', and note that it is not auto-capitalized.
- As a stronger challenge, type in 'i'. Note that 'i' is neither auto-capitalized nor auto-corrected to 'I'.

Reviewers: jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D9697