Remove luminous library, content plugin, and render markdown test page based on GitHub's API

This seals the deal on using [GitHub's Markdown API](http://developer.github.com/v3/markdown/) as our Markdown parser instead of using the Elephant Enhanced Markdown and Luminous library.  Don't get me wrong, it's great code and has some better options than GitHub's own parser does right now, but there are three wins here:

* Consistent parsing of user input between GitHub and the tracker application
* Less code to maintain
* The markup is saved into the database versus requiring to be parsed each time the page is loaded

Before merging this into master, I want to give the chance for anyone watching to voice opinions yay or nay.  I know I kinda jumped the gun with my commits yesterday basically changing the parsing everywhere else, but commits can be reverted if need be :wink: