[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
* Add a var for the cubic bezier curve value
I agree that it's more obvious what it is - but it's not easy to remember. I feel like lines 34, 35 and 36 should have a convention which can be logically followed to figure out what a default var would be for any value. Eg:

```CSS
border-radius: $border-radius;
box-shadow: $box-shadow;
transition-duration: $transition-duration;
```

Would make sense to me. That way whenever I'm writing new SASS, I know instantly what the var will be for the default value without having to remember anything.

Thoughts?