Support hot reload if service worker is already registered

[seejamescode]

If you happen to run your production server on localhost (to test the build), the service worker is registered as desired.

However, you the service worker then gets in the way of react-scripts start/yarn start's hot reloading.

I will make a PR recommending an edit to the HTML if statement that is working for us:

<script>
    if (
      'serviceWorker' in navigator &&
      window.location.hostname !== 'localhost'
    ) {
      window.addEventListener('load', function() {
        navigator.serviceWorker.register('/service-worker.js');
      });
    }
  </script>