Allow private media to be downloaded with authentication

This PR adds a view to the template project that can be used to download files with authorisation.

A `Dockerfile` is added too so the uWSGI/Nginx integration can be tested.
I have tested this with the docker container and seen it working so I'll leave it for now. It probably depends on whether your `X-Accel-Redirect` value starts with a slash (they don't in this implementation).