Add support for LaTeX

Summary: Using a 3rd party lib, https://github.com/ProfFan/phabritex

Test Plan:

• Go to a wiki page
• Add the following text

katex {{{
\begin{bmatrix}
a & b \\
c & a
\end{bmatrix}
}}}

Subscribers: soubies

Differential Revision: https://c4science.ch/D133