New nav, TBD /about page

This matches the nav as in the mocks in https://github.com/18F/foia-hub/issues/176#issuecomment-62955346, and uses the restructuring as specified in #212. #212 isn't finalized, but this can always be changed later if need be. I wanted to get this part done before moving on to the main landing page.

It restructures the nav area to be inside a main `<header>`, which contains a `<h1>` logo, and a `<nav>` container that contains a `<ul>` for the actual list items. It does a semi-ugly relative/absolute positioning thing to get the nav's list items down to the bottom of the header. The header is of a fixed height for this reason.

So that the "About this Project" thing went *somewhere*, I made a TBD page for it. I can fill it in to solve #211 whenever there's copy.
We had some leftover classes from when we used to use [Pure](http://purecss.io/).