KaTeX escapes its own input and we trust it to generate safe HTML as
output. Passing HTML generated by KaTeX through ngSanitize can mangle
the result.  eg. The formatting of the infinity symbol is corrupted when
using this test block for example:

  $$\lim_{x \to \infty} \exp(-x) = 0$$

This commit rewrites the rendering logic to only sanitize the markdown
output from Showdown.

This additionally fixes an issue where text that _happened_ to also
be valid markdown in the rendered math would be rendered as markdown,
which is not what we want.