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 &
Could we remove the `layout-` from this class? I.e. `wsk-navigation`instead of `wsk-layout-navigation`. The latter ties the navigation fairly explicitly the the layout. If this is a specific instance of the nav that cannot just be called `wsk-navigation`, perhaps we could have `wsk-navigation--main`…? I don’t have full context here, so you may well have perfect reasons for this approach that I’m just not aware of :)