Verification: Switch class-based Sass @extend rules to use Sass placeholder syntax

This work aims to move any class-based (e.g. ".ui-fake-link") Sass @extend into the new placeholder (e.g. %ui-fake-link) syntax (introduced in Sass 3.2). This work does not touch the following:

* any classes that are actively being used in HTML elements
* any extends used elsewhere in the lms Sass architecture/other LMS views

Since we want to repurpose some of this work for future LMS stuff, it made sense to get this stuff into shape now. This will cut down selector specificity and rendered CSS duplication/redundancy significantly.

For more info around the differences and some background, see - http://blog.teamtreehouse.com/extending-placeholder-selectors-with-sass