add DOM struncture for the new nav

Add new navigation structure behind a switch. This will allow us to iterate faster over design. When NewMobileNavSwitch is on:
- the old navigation will not be rendered
- the hamburger menu will be hidden for both mobile and non-mobile breakpoints.
- the new navigation will be rendered on all breakpoints
I'm surprised this passes scss lint (grunt validate:sass). Let's variabilise this coulour.