Cleaning up layout SCSS

- Make sure as much as possible is behind optional classes
- Rename everything to follow style convention
- Add new class for JS binding
- Rearrange SCSS to take advantage of nesting with &
Nope, that makes sense, thanks! Historically, this has always been a part of this component, so I hadn't considered that option.

I'll leave it in this file until we find a need for it elsewhere, though. We'll probably look at it as part of a future globalization pass (I seem to recall other components had some bits that could get made global).