Appropriate settings and code to manage storages for static site generation

[veselosky]

When running in production mode and generating static sites, we want all storages to be S3-compatible buckets. However, test automation should be configured differently, probably should use local files for performance. Local dev may want one or the other depending on circumstances.

• Create logic in the settings.py to switch configurations per environment
• Update example.env with all settings needed to configure S3 storages for dev and prod
• Create a new storage class that combines features of S3Storage and ManifestStaticFilesStorage (using ManifestFilesMixin I assume)