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). 
Yeah! That would be great.