Asset store for uploaded files

[goto-bus-stop]

For emoji and avatars, we need to be able to upload and store files. That could probably use some shared machinery, so all the files end up in a predictable place. We need to make sure that it's easy to use nginx to serve the files instead of having to go through node.js.

It should also be used for custom site logos and images used in the About page description.

This should maybe be backed by abstract-blob-store?