Allow themes to inherit from the default navigation bar and override
pieces of it, including the main logo, the links that display to the
right of the logo, and the links inside the dropdown menu (with the
exception of the `Log Out` link.

In addition, this adds an empty block at the very top so that themes
can place a branding bar at the top of the page. (Stanford identity
guidelines require this: see https://identity.stanford.edu.)