Configuration for public/private storage policies

[benjaoming]

In #144, we moved the configuration of MinIO storage bucket policies to a process related to initialize the web container. This was a kind "configuration as code" chosen over the manual setup from before.

Some buckets are private, depending on the environment. This should be expressed as configuration somewhere.

https://github.com/readthedocs/common/pull/144#discussion_r898097442 https://github.com/readthedocs/common/pull/144#discussion_r898097826

This issue might be solved in a way that only affects the docker setup, and if possible/necessary can be integrated for other deployments as well.