LP#1942220: add 'Disencumber' action for PO items

LP#1942220: add 'Disencumber' action for PO items

This adds a button to, upon user confirmation, change the
fund debit for a PO item to a zero-value encumbrance.

This button is available for a charge only if:

- the PO item is attached to a fund debit that has
  no invoice entries or items attached
- the PO is activated but not cancelled
- the fund debit is not an expenditure
- the debit amount is not already zero

The purpose of this button is to clean up encumbrances for
miscellaneous charges on invoiced POs that have not been
linked to invoice items.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>