Upload from URL

[Floppy]

Rather than attaching local files, it would be nice to be able to upload straight from a URL, particularly for single-file uploads like in #2032.

Uppy has nice support for this with https://uppy.io/docs/url/, but that needs their Companion app running server side. I think Shrine can pretend to be Companion, but I'm not sure how, so I've asked for advice at https://github.com/shrinerb/shrine/discussions/693.

Hopefully we can work it out!