Improving layout classes

This update makes it so that switching between our two main layouts as
simple as switching a class, and adding the bleeding background style as
simple as adding the (now shorter-named) `bleedbar` class.
Merge from upstream

Conflicts:
	static/css/main.ie.min.css
	static/css/main.min.css
Reorder classes
Better naming and other stuff

- Mo betta naming following discussion yesterday
- Ensures small-tablet-plus layouts have 30px page margin
- Makes demo-banner more distinct from the rest of the page, improves
  its layout at narrow widths.
Change how page margins are figured out
Merge pull request #14 from Scotchester/gh-pages

Improving layout classes.