Summary:
Previously, absence of `GRIST_DOCS_S3_BUCKET` was equated with absence
of external storage, but that is no longer true now that Azure is
available. Azure could be used by setting `GRIST_DOCS_S3_BUCKET`
but the alternative `GRIST_AZURE_CONTAINER` flag is friendlier.
Test Plan:
confirmed manually that Azure can be configured and
used now without `GRIST_DOCS_S3_BUCKET`
Reviewers: alexmojaki
Reviewed By: alexmojaki
Subscribers: alexmojaki
Differential Revision: https://phab.getgrist.com/D3448
