File upload follow-up

[RadoRado]

Related to #66

What else needs to be done:

• Add support for CloudFront CDN (django-storages config) - lets call this "media CDN"
  • https://django-storages.readthedocs.io/en/latest/backends/amazon-S3.html#cloudfront
• Add support for static files going to S3 bucket & also being served by a different CDN - lets call this "static CDN"
  • https://django-storages.readthedocs.io/en/latest/backends/amazon-S3.html#overriding-the-default-storage-class
• Add tests for a lot of things in the files folder
• Update README, perhaps, after the blog post is done
• Explore the "callback" option for the S3 direct upload, as an alternative to manually calling the finish API
  • Look for "success_action_redirect" here - https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html

[RadoRado]

Addressed some of the points in #166