Upgrade to new major release 4 of React Router

This also introduces a new PrerenderableRouter component, which adds the correct router depending on whether we're rendering on the server (in CI) or client-side. Since the test setup emulates a browser-like environment, testing that properly is challenging - so I skipped that in our coverage measurements.

(Unfortunately, the regular way of adding /* istanbul ignore else */ did not work. I'm not quite sure why, but I left it in to be able to retain the comment explaining that it's ignored and why.)

Closes #68 (closed)