CSS layout architecture improvements

- [new] `l-` prefix for header and footer layout modules (making them easier to search through the codebase)
- [new] remove old grid system css, since now we use guss-grid-system 
- [new] Layout files organisation (header / footer / helpers)

Bonus: thanks to this change, we no longer have any ID selectors in our CSS codebase :)

![ ](http://media1.giphy.com/media/Eiu0LnoV0qdC8/200.gif)