Allow deleting books and copies
From Ticket#2019072871000133
A way to delete copies of books that have accidentally been created is needed.
It is important that no copies that actually exist(ed) are ever deleted - they should instead be marked missing retired.
Accidental deletion is not an issue if we limit deletion to books that have never been borrowed. We could still add a JavaScript based confirmation modal, and can skip server-side enforcement of the confirmation because books that have never been borrowed are easy to restore.