Add support custom domain for S3 storage

Related issue: N/A

This merge request will gives availability to server owner who using S3 as a CDN, without proxying files to server.
It will helps reducing traffic for server, and also improves file transfer speed for users.

Here is original document about environment variable what I inserted:
https://django-storages.readthedocs.io/en/latest/backends/amazon-S3.html#cloudfront

This Merge Request includes:

• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)