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). 
You should go back to the way you had before with this as a requirement listed in assets.yaml. You can just provide this URL as the contents of the bundle and that will work fine. This way, it's easy to override the same as any other asset if someone wants to load MathJax from elsewhere.