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Now I raise a django builtin permission denied instead of writing a custom HttpResponse

This is not implemented using a `def head()` in the view, as the HEAD request
should return all headers that the GET request would send. Hence,
updated the server logic to provide a proper HEAD request.

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

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