Navbar visual tweaks

This pull request includes a bunch of small visual tweaks to the navbar, such as:

* add some padding to the bottom of the search box to make it look more even
* unstick the footer so it's on the bottom of the page, instead of fixed always on screen
* give the footer some breathing room, so it feels like it anchors the page
* change to a more modern blue (feel free to keep it brown though!)

I left it mostly in the bootstrap overrides file so that the big css cleanup later can put them in the right place.