Display mathematical notation properly

Implements feature request #1200 suggested by @aronasorman. Fractions, exponents, derivatives, etc. are now displayed in their proper form. You just put backticks around the formula to mark it for transcription.

Some examples for testing:

/math/arithmetic/exponents-radicals/negative-exponents-tutorial/v/negative-exponent-intuition/
/math/differential-equations/first-order-differential-equations/homogeneous-equations/
/math/calculus/differential-calculus/der_common_functions/v/proof--d-dx-e-x----e-x/
/math/calculus/integral-calculus/u_substitution/v/2-ln-x--x--antiderivative-example/

PS: The MathJax library stores its font data in a lot of tiny files, and IMHO it takes too long to check each file individually. Could we add an exception to the static file checker to just skip those folders?