Undo an import

Removes a draft that should never have landed — the way out of an import whose column read-back arrives AFTER the dataset, the dashboard and the row already exist. Removes the catalog row and the dashboard it stood up, in that order, so a failure halfway cannot leave a row pointing at nothing. Refused for a live template (unpublish stops it being offered without destroying anything) and for a row the catalog does not own. A row people have applied may be discarded: an apply copies, so their boards keep working — what goes is the template, its preview board and the record that it was used. The DATA is released last, and only if no other template still points at it — it is content-addressed, so two templates carrying identical bytes share one snapshot row.