Can't Delete an Unpublished Instance

Summary: Create an Instance of a Publishable Model. Without Publishing it, try to Delete. You will get an error saying that it is a Thing you aren't allowed to see.

The problem is that DeleteThing isn't Publication-smart; it's trying to run locally in SpaceCore, and can't find the Instance, so it's returning the "UnknownPath" error.

Fixing this isn't too hard in theory: we need to detect that it's a pre-Publication Instance in DeleteThing, the same way we do in ChangeProps, and forward it to the InPublicationActor. That already knows how to handle deletions (since that is what happens when you Publish an Instance), so it's likely to Just Work.

The only complication is that, since we provide no visibility into the InPublicationHistory yet, and have no Undelete API call for it, Deleting an Unpublished entry is effectively non-undoable. So it demands that we reinstate the "are you sure?" confirmation, just on this annoying little edge case.