Don't include main nav when none specified

* If navbar_right and navbar_items content is not set, don’t include
the responsive menu and the navigation containers
Prevent navbar height from collapsing when empty

* Necessary when there are no navbar_items or navbar_right, for
instance when using Signon’s Sign in screen
Merge pull request #28 from alphagov/empty-navbar

Tidy up navbar when no navigation specified