Studio: 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 of the older/antiquated Sass rules in _mixins-inherited.scss 
* does not refactor any other Sass elements/make corrections

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