Cleanup modules Sass

References #4107

This is the first step in a larger project to reformat our Sass. Each 'chunk' will reformat a directory in `core/client/assets/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
* Improve var names and add a couple new ones