Add option to serve S3 media from a custom domain

[mbertrand]

What are the relevant tickets?

Related to https://github.com/mitodl/hq/issues/9046 and mitodl/ol-infrastructure#3979

Description (What does it do?)

• Adds a new setting AWS_S3_CUSTOM_DOMAIN
• If the above is not blank, the MEDIA_URL setting is updated to include the domain

How can this be tested?

• Set MITOL_USE_S3=True if it isn't already.
• With AWS_S3_CUSTOM_DOMAIN blank/null (default), everything should work as before
• In Django Admin, look at the details for one of the Testimonials. If MITOL_USE_S3=True was already true, the avatar and banner images if not blank should have S3 urls. If they don't, upsert a new image. When done it should have an S3 url.
• Set AWS_S3_PREFIX to something like "mymedia/" and restart
• Go back to an Attestation object and upload a new image. Check that the S3 url for it has "mymedia" as part of the URL right after the domain
• Set AWS_S3_CUSTOM_DOMAIN to something like fakefastllycdn.com and restart the containers
• Go back to Django Admin and look at the urls for the avatar images, they should start with https://fakefastllycdn.com

[shanbady]

👍 looks good

small question i had is why not just make the MEDIA_URL itself configurable via env var instead of creating a new flag

[mbertrand]

@shanbady I decided to modernize how this is set since Django 4.2 - https://django-storages.readthedocs.io/en/latest/backends/amazon-S3.html

and updated the testing instructions