LP#1319519 Add to PO limits to new/pending PO's

Limit the auto-select output of the PO name text box when adding
records/lineitems to existing PO's to PO's with the "new" or "pending"
statuses.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>