Accession deletes

A first go at deletions for accession records.  I'll review this myself tomorrow, but more eyes would be good if y'all have time.  When you delete a record, what should happen:

  * Nested records get deleted
  * Relationships between the record being deleted and other records get deleted (the other record remains, though)
  * Any external IDs pointing to this record get deleted with it

And the whole thing is recursive, so if a nested record has related records or external IDs, those go too.