Header nav doesn't indicate active page

[domoscargin]

The Kit uses a header with navigation, but doesn't indicate active pages.

We should ensure that pages in the navigation menu are flagged as active as appropriate.

There are 3 links in our navigation menu, but plenty of sub-pages - so we also need to answer the question of whether we treat the menu items as "categories" and highlight them for all sub pages, or whether we only highlight them when on those specific pages.