Add support for S3-backed image storage

assets/nsfw/README.md

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+# Bundled NSFW model
+
+`model.onnx` is the pretrained model distributed with
+[Fyko/nsfw v0.2.0](https://github.com/fyko/nsfw/releases/tag/v0.2.0), based on
+[GantMan/nsfw_model](https://github.com/GantMan/nsfw_model).
+
+Source: https://github.com/fyko/nsfw/releases/download/v0.2.0/model.onnx