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).

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 does 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/