Nav restructure

This makes possible to define separate list of navigation items to be displayed in top bar vs global navigation (mega nav), as per editorial request. In order to make this possible, I've introduced the briefNav, a list of nav items to be displayed in the top bar, per edition.
Also, the content of the navigation has been restructured.
![](http://media.giphy.com/media/6q0yzgHx5YYZW/giphy.gif)