Navigation for / and /top/:page should highlight the "top" tab in header

I don't see a way to pass a custom method to `` from React-Router for denoting it's active classes.

Idea 1:
Use a custom `` component to derive when either url pattern is matched and apply via standard classNames

Idea 2:
Modify React-Router to accept custom methods for determining active states.

Idea 3:
Move away from React-Router entirely and implement a custom router. (Likely don't have time for this).

No files, tests, or entry points are named. Inspect the header navigation and route definitions first, then determine how the existing React-Router setup handles / and /top/:page; done means both routes visibly mark the “top” tab as active.

Written by the indexing model from the issue text.