Whitelist files that can be embedded through url-loader

Created by: pugnascotia

Change the current catch-all loader to use file-loader instead of url-loader, and exclude common image file extensions. Add another url-loader for images, configured identically to the original catch-all loader.

Part of issue #1293 (closed).

Testing

1. Create a demo app with ./tasks/cra.sh my-app-test
2. cd my-app-test
3. Add a 1K PNG, a 50K PNG and a PDF to ./src
4. Import each resource into App.js and include them in the page

import smallPicture from './small.png';
import largePicture from './large.png';
import someDoc from './someDoc.pdf';

// ...
<p><img src={smallPicture} alt="small" /></p>
<p><img src={largePicture} alt="large" /></p>
<p><a href={someDoc}>Here's a PDF</a></p>

5. Start dev mode and check that:
   • The small image is embedded
   • The large image is referenced
   • The PDF is referenced

yarn start

7. Build a production bundle:

yarn run build

8. Check that the production build works as expected by serving it and browsing http://localhost/

docker run -it --rm -v $PWD/build:/usr/share/nginx/html -p 80:80 nginx:alpine