Azure private storage

[bufke]

Description

Adds support for Azure in django-private-storage

Hacky things

• I didn't feel like overriding private_storage.appconfig so I'm reusing PRIVATE_STORAGE_S3_REVERSE_PROXY which is silly.
• AzureStorage get_modified_time 'ContainerClient' object has no attribute 'get_blob_properties'  jschneier/django-storages#1131 should be fixed upstream. We could use this as the regular AzureStorage class instead or only on private.
• We could contribute PrivateAzureStorage upstream to django-private-storage but I would need to support all the overrides they support (which I don't really understand why that's necessary).

Weird things

• PrivateS3BotoStorage overrides a bunch of defaults of S3BotoStorage but I don't know why. Most of the overrides don't seem relevant. It's already being set in the base class. The claimed "differs from base class" are not accurate. AWS_PRIVATE_DEFAULT_ACL is None by default which uses the S3 default of private.

Related issues

Related to #3794