Fix MathJax plugin to work with our editor.

Summary:
Also, fix it to work at all. Whoever originally wrote this thing should
have their keyboard taken away from them.

I decided we coule make this thing a lot simpler and faster by using the
React TeX component rather than creating an iframe, downloading MathJax
inside it, and communicating with cross-document messaging... for
everything we need to typeset.

Question for reviewers:
What's the canonical version of the content? I see both `content` and
`htmlContent` in my PUT request (also the "translated" version of both -
why?). `htmlContent` has my changes but `content` doesn't... Is that
okay?

Test Plan:
* open "Common Questions about Dates" to edit
* click the sigma button and see a LaTeX editor pop up
* change stuff in the editor and see it instantly reflected below
  - I tried to make this appear larger with the `font-size: 150%` thing
    but it seems to have made no difference. Suggestions appreciated.
* click "OK", see the latex I entered rendered in the page
* double click the rendered latex, the editor opens back up
* edit it, confirm that the in-page version changes
* reopen, confirm that my latex is the edited version
* reload the page, confirm that my latex is still there
* drag it elsewhere using the drag handle, somehow it turns into
  unicode, despite CKEditor's promises to the contrary:

> All widgets can be dragged inside the editing area of CKEditor and
> dropped wherever you want to place them. And since the widget
> structure is immutable, there is no chance that the widget will become
> corrupted in the process or otherwise fall apart!

Any suggestions on how to fix that are welcome!

Reviewers: pamela, tom

Differential Revision: http://phabricator.khanacademy.org/D9353