Add new S3 storage class, PrivateS3BotoStorage and PrivateEncryptedS3BotoStorage

This class can now be selected with the new `PRIVATE_STORAGE_CLASS` setting.

private_storage/storage/s3boto3.py

+from storages.backends.s3boto3 import S3Boto3Storage
+from storages.utils import setting
+
+
+class PrivateS3BotoStorage(S3Boto3Storage):
+    """
+    Private storage bucket for S3
+    """
+    # Since this class inherits the default storage, it shares many parameters with the base class.
+    # Thus, redefine the setting name that is used to read these values, so almost all settings are not shared.
+    access_key = setting('AWS_PRIVATE_S3_ACCESS_KEY_ID', setting('AWS_PRIVATE_ACCESS_KEY_ID', S3Boto3Storage.access_key))
+    secret_key = setting('AWS_PRIVATE_S3_SECRET_ACCESS_KEY', setting('AWS_PRIVATE_SECRET_ACCESS_KEY', S3Boto3Storage.secret_key))
+    file_overwrite = setting('AWS_PRIVATE_S3_FILE_OVERWRITE', False)  # false, differ from base class
+    object_parameters = setting('AWS_PRIVATE_S3_OBJECT_PARAMETERS', {})
+    bucket_name = setting('AWS_PRIVATE_STORAGE_BUCKET_NAME', strict=True)
+    auto_create_bucket = setting('AWS_PRIVATE_AUTO_CREATE_BUCKET', False)
+    default_acl = setting('AWS_PRIVATE_DEFAULT_ACL', 'private')  # differ from base class
+    bucket_acl = setting('AWS_PRIVATE_BUCKET_ACL', default_acl)
+    querystring_auth = setting('AWS_PRIVATE_QUERYSTRING_AUTH', True)
+    querystring_expire = setting('AWS_PRIVATE_QUERYSTRING_EXPIRE', 3600)
+    signature_version = setting('AWS_PRIVATE_S3_SIGNATURE_VERSION')
+    reduced_redundancy = setting('AWS_PRIVATE_REDUCED_REDUNDANCY', False)
+    location = setting('AWS_PRIVATE_LOCATION', '')
+    encryption = setting('AWS_PRIVATE_S3_ENCRYPTION', False)
+    custom_domain = setting('AWS_PRIVATE_S3_CUSTOM_DOMAIN')
+    addressing_style = setting('AWS_PRIVATE_S3_ADDRESSING_STYLE')
+    secure_urls = setting('AWS_PRIVATE_S3_SECURE_URLS', True)
+    file_name_charset = setting('AWS_PRIVATE_S3_FILE_NAME_CHARSET', 'utf-8')
+    gzip = setting('AWS_PRIVATE_IS_GZIPPED', S3Boto3Storage.gzip)  # fallback to default
+    preload_metadata = setting('AWS_PRIVATE_PRELOAD_METADATA', False)
+    url_protocol = setting('AWS_PRIVATE_S3_URL_PROTOCOL', S3Boto3Storage.url_protocol)  # fallback to default
+    endpoint_url = setting('AWS_PRIVATE_S3_ENDPOINT_URL', None)
+    region_name = setting('AWS_PRIVATE_S3_REGION_NAME', S3Boto3Storage.region_name)  # fallback to default
+    use_ssl = setting('AWS_PRIVATE_S3_USE_SSL', True)
+
+
+class PrivateEncryptedS3BotoStorage(PrivateS3BotoStorage):
+    """
+    Enforced encryption for private storage on S3.
+    This is a convience option, it can also be implemented
+    through :class:`PrivateS3BotoStorage` by using the proper settings.
+    """
+    encryption = True
+    signature_version = PrivateS3BotoStorage.signature_version or 's3v4'