render() {
    return (
      <h1>
        <FormattedMessage {...messages.header} />
      </h1>
    );
  }
NotFoundPage

This is the page we show when the user visits a url that doesn't have a route

NOTE: while this component should technically be a stateless functional
component (SFC), hot reloading does not currently support SFCs. If hot
reloading is not a necessity for you then you can refactor it and remove
the linting exception.