Warn when the website is in quirks mode

Summary:
KaTeX doesn't work correctly in quirks mode. Warn in the console and disable
rendering if that happens.

Test Plan:
 - Make sure the test still loads and renders math
 - Make sure a warning is thrown when the doctype is removed, and no more math
   is rendered
 - Make sure the tests pass both on the web and with `make test`

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13192