Refactor server rendering with `react-router` <StaticRouter>
and update client to use `react-router-dom` <BrowserRouter> and
other components.

Server rendering documentation:
https://reacttraining.com/react-router/web/guides/server-rendering

Use <StaticRouter> context object to deal with server redirect
or 404 response.

Create new catch-all (404) route that renders <NotFound>.

react-router@4.1 causes the app to break with the error:
```
Uncaught Error: Calling PropTypes validators directly is not
supported by the 'prop-types' package.
Use 'PropTypes.checkPropTypes()' to call them.
Read more at http://fb.me/use-check-prop-types
```