Store origin information for exchanged resources #1272

When exchanging resources between TeSS instances, the receiving instance should store some metadata pointing back to the origin instance.

This could just be a URI for the original or something more involved.

Then we can:

• Point back to the original resource (because maybe the original resource has been updated and the "copy" is stale)
• Resolve conflicts that may occur with "transitively" exchanged resources (i.e. exchanged resources which were themselves exchanged from another instance), or choose to deliberately not exchange already-exchanged resources.