Move navigation ready for the new nav bar.

This will need the new version of slimmer before we can use it.
Update slimmer and ready for new navigation

- New navigation doesn't need the old CSS so deleted it.
- No longer needs the toggle js
- Add in a top-bar element. This was simpler than trying to attach to a
  single element.
- Added some overides for the inside gov brand colour.
Merge pull request #421 from alphagov/new-nav-bar

Tracker: https://www.pivotaltracker.com/story/show/46815487

Implement new nav bar