markdown: Fix issues with new markdown styles.

Based on feedback from the /r/modnews post:

* font sizes are now em based, and respect browser default font size preferences
* grey text for blockquote and del elements is darker, meets AA accesibility guidelines
* bold/italics style combinations fixed
* removed larger font sizes for wiki pages
* reduced a lot of the margins and altered some of the line-heights.  pages should be
  more compact vertically

This should make the markdown styles respect users' browser
preferences better.