[WIP] Sass cleanup

References #4107

This is the first step in a larger project to reformat our Sass.
The main changes here are:

* Rename `helpers` -> `modules`
* Rename `lib` -> `vendor` (a common practice from JS dev)
* DRY up icon styles by using a placeholder selector for base icon styles
* Add new comment block at the tops of files explaining what's what and list the main parts of the file
* Removes unused and unnecessary mixins & variables
Styles should be capitalised, as it is referring to "Styles" as a naming convention for code structure within Ghost