Added setting for Django Storage to use Cloudfront for S3 files (#2711)
Added setting for Django Storage to use Cloudfront for S3 files (#2711)

Added the `AWS_S3_CUSTOM_DOMAIN` setting in order for S3 assets
managed via Django Storages library to be served via CloudFront.
mitodl/micromasters,mitodl/micromasters,mitodl/micromasters,mitodl/micromasters