This is required in some AWS regions.
The right long-term fix is to move to boto3 which doesn't have this
problem.
Allows us to downgrade the priority of #9376.
This moves the documentation for this feature out of
prod_settings_template.py, so that we can edit it more easily.
We also add a bucket policy, which is part of what one would want to
use this in production.
This addresses much, but not all, of #9361.
Tim Abbott