[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
I'll shorten the name (same for below).
I think it definitely should be a var, as its used in 9 places, many of which are applied at the same time. Any change to one and not the others would totally ruin the effect. It's safer to have one place to edit instead of 9.