Studio Sass Mixin/Extend Re-Org

This pull request organizes the Sass mixins/extends used throughout Studio. It includes:

1. prefixes each extend with set purpose type for better understanding/scanning/application (cont = content, ui = user interface)
2. a re-org of extends Sass file used by purpose (UI, content, etc.)
3. references the new extend names in rules where their previous versions were called 

The hope is that this can be a convention going forward with things like modules (m-*) and layouts (l-*) coming next.