Navigation v0.2

This is the first iteration of a responsive navigation. I'm afraid a lot of files have changed and includes some radical refactoring.

### Includes:

- Complete CSS tidy up and refactor of the ```.nav``` object
- HTML separation of mobile sections drop-down and wider horizontal nav (ask me for why we chose this route)
- Refactor of navigation data model and scala Navigation class
- Search now top level item with own drop-down (including required JS changes)
- Top stories CTA refactor and new icon

### Doesn't include:

- Secondary sub-section navigation (awaiting design sign-off)
- Hover and active states for global nav (awaiting sign-off)
- Search at wider breakpoints (needs further refactoring)


Hopefully this will now enable us to experiment with a few different versions as discussed over the coming week.

@commuterjoy @ahume @stephanfowler @ironsidevsquincy @mattandrews please review :smile: