[FEATURE] expose navigation state values

[acharliekelly]

The current Dashboard Header component gives control of the nav tabs to DesktopNav (and presumably also an as-yet released MobileNav), which stores them in internal state (which is fine, since so far the header has been purely decorative). But to make other parts of the app react to tab clicks, we either need to lift that value up to the Dashboard page, or use some other nifty tool to do the same thing.

Also, are we making Search, Profile, and About into fully-navigable pages, or just swapping the content of the dashboard?