Remove redundant Tumblr controls hack.
Refactor navigation-related stuff.

  - Simplify navigation-related stuff.
  - Add some documentation and try out quasi-qualified CSS selectors.
    (See <http://csswizardry.com/2012/07/quasi-qualified-css-selectors/>
    for more infomation on quasi-qualified CSS selectors).
  - Move clear-fix rubbish into a mixin.