Move Alignment and Accessibility helper classes to their own less files

This also removes IE7 support from the clearfix
Move the skip link to the top of the page and link it to #content instead of #main

A previously version of Cakifo had #main as the wrapper instead of
#content but the link never got updated
Fix skip link focus

The JS is from _s and the default themes.
Don't outline the content when focused
Use .screen-reader-text instead of .assistive-text as it's easier to understand

The old class is kept in the CSS in case a child theme uses it.