LP#1436906 clean up PO direct charges on delete/cancel

LP#1436906 clean up PO direct charges on delete/cancel

1. Deleting a "direct charge" from a PO removes the linked fund_debit if
the fund_debit has not been paid (i.e. it's still encumbered).

If the debit has been paid, the charge cannot be removed, unless/until
the debit is re-encumbered by un-invoicing the charge.

2. When a PO is canceled, fund_debits linked to PO items are removed.
As above, if the debit has been paid (invoiced), the PO cannot be
canceled, unless/until direct charge debits are un-invoiced.

Signed-off-by: Bill Erickson <berickxx@gmail.com>