GitLab

This is needed to fetch content from S3 without needing a full_path

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

The default `private_storage.storage.PrivateStorage` class now points to
the class that was imported through the new setting, theirby preserving
backwards compatibility.

- Added get_path()
- Added get_private_file()
- no longer depend on `self.kwargs['path`]`.