Studio: renames and re-orgs app-wide Sass extends
Studio: references newly renamed app-wide Sass extends
Studio: adds further comment/prefix revisions to Sass extends
Studio: renames and simplifies .cont-trucated Sass extend
Studio: renames nav dropdown pattern and corrects references
Studio: updates Sass mixins/extend references based on textbook/Sass cleanup
Merge pull request #319 from edx/talbs/studio-sassmixins

Studio Sass Mixin/Extend Re-Org