This change accomplishes several things:

- Use a <nav> element for the header links.

- Change the frontend code to delay setting up the routes until the basic

  elements of the page are fetched from the server.

- Added a spinner while the main data is loading.

- Added the admin link (which currently doesn't do anything, but that's next

  up).

- Changed various URLs so that endpoints that return JSON are all under /api/

- Indentation fixes.

Verified that everything still works as expected.