CONCEPTUAL: Support custom config of Highlight language support

Map a Controller to /javascript/highlight.js so we can dynamically
return a highligh.js version based on configured language support.

Configuration of Highlight can be found under Admin > Settings > Highlight.

*DO NOT PULL UPSTREAM*

Would this approach be something to consider for custom language support in highlight configured via Discourse admin settings? With this it's up to the admin to choose specifically which of the n languages to support. 

![screenshot from 2014-10-25 19 45 54](https://cloud.githubusercontent.com/assets/132158/4780579/fc842bde-5c6e-11e4-8d87-d3bcd22b2c54.png)


Missing currently: 
* Some sort of caching of the dynamically generated/joined highlight.js file in the controller?
* Compression of the new highlight.js parts: highlight.js + languages
   * Currently require custom source build of highlight.js
This is actually a massive perf problem. 

1. You are going to need some sort of hash or global version number so you can set expiry to forever
2. You should use sendfile to ship this. 
3. You should handle concurrency here, ideally. 

Have a look at how we do avatars, also this should live in that same controller.