Remove the double top header, moving global nav to footer

This MR removes the double top header which has confused several people, moving the "global" org-wide navigation to the footer instead. The remaining site top nav is made sticky during scrolls.

By doing so, we have also removed the black bar from the top of the page, which gives the website a more open, plain look. This can go both ways as the plainness can give of an unfinished impression, but at the same time it reduces the formality that the black bar was giving, making the platform friendlier in atmosphere.