More robust styles for sidebar links

Prevent them from being easily overridden by a global `a` declaration.
Fix codeblock background helpers.
Tweak cg-Page children lists and paragraphs.
List styles
CoffeeScript optimization
Use custom Marked -> React transformation

react-marked had some problems with parsing and doesn't make much sense because marked should be used as a library, not be forked wholesale. Of course, this way we're very tightly bound to marked, but a more flexible (array-based instead of string-based) parser should be part of marked.