Markdown subset support

Fixes #1124

This PR replaces StackExchange's PageDown with [marked](https://github.com/chjj/marked) which supports [GitHub Flavored Markdown](http://github.github.com/github-flavored-markdown) (GFM). It also adds highlight.js for syntax highlighting of most languages.

GitHub issues, PRs and comments all use the full GFM parser (block and inline elements), while content (chat messages, topic, etc.) only allows inline elements, like ``code``, `_italics_` and `**bold**`. This makes sense while there still isn't support for multiline chat messages. Pasted text is not parsed for markdown. I've disabled markdown links, so you're not able to do `[google.com](some.malicious-site.com)` :smile:

Both chat and GitHub issues/comments parsers use sanitation, so it should be safe from XSS.

You can test this out on http://jabbr-markdown.azurewebsites.net/