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.

- 2-1 layout with bleedbar: http://scotchester.github.io/cfgov-refresh/blog/landing.html
- 2-1 layout plain: http://scotchester.github.io/cfgov-refresh/blog/entry.html
- 1-3 layout: http://scotchester.github.io/cfgov-refresh/newsroom/landing.html