[DO NOT MERGE] Header/footer/nav refactor

I suspect this is one of the more complex bits of the CSS, and I'd like to get it a bit cleaner before we start scaling up. At the moment there's various unused rules, duplicate ID attributes, and generally the nav, column-nav stuff (which I like) is all mushed up with sections, headings, logos, etc...

This check-in does a few things:

* Splits navigation.scala.html into `headerNavigation.scala.html` and `footerNavigation.scala.html`. Working in that file was most unenjoyable with its if statements all over the place.
* Extracts a 'brand-bar' component. This is the bar that runs along the top and in the footer contains the logo. It was originally called 'header', which is confusing because it could be used anywhere. It may or may not contains some navigation components. I may abstract brand-bar-nav into a stand-alone thing as well.

There's more changes to come, and the codes in a bit of a half state, but would appreciate early feedback.