Allow users to attach their own storage

[Floppy]

We support multiple libraries, on different systems. There's no reason that a library on a public server couldn't be added by a particular user, allowing them to connect up models hosted on (for example) a personal google drive.

The user should be default owner for anything on that library, and we should have per-library path templating as well.

This would mean that a user of a public server, say 3dprint.social, could point at their own google drive or similar, and publish their models via the UI while retaining full control of the actual storage.