Closes#2522
To reduce the strain on the API service, we moved the media file serving
to the Nginx web server. The API is still handling the authentication,
but delegates the serving using the `X-Accel-Redirect` header.
BREAKING CHANGE: The media file serving is now handled by Nginx instead
of the API service. The `storage.path` field is now used in the Nginx
configuration, so make sure to update the Nginx configuration file if
you change it.
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