Add MathJax CDN

This PR adds MathJax, so we can render math written in LaTex or MathML:
![hypothesis](https://cloud.githubusercontent.com/assets/521978/4208235/51973c86-385e-11e4-8f9f-747d1d0d8426.jpg)

Give it a try on: https://mathjax.dokku.hypothes.is/ 
Note: the markup for math consists of two dollar signs: `$$LaTex or MathML here$$`

This PR is an alternative to including the very large MathJax library in our vendor folder (as implemented in https://github.com/hypothesis/h/pull/1480). 
So there are way too many markdown elements, but on my view-sort-overhaul branch there's been some cleanup at least of the rendering cycles. If I take your changes here and put them on that branch then things render on save perfectly fine. The preview still doesn't render. The reason is that MathJax cannot do proper layout when the element is hidden.

Let's pair on this sometime soon after that branch lands.