LP#1635354 Invoice close date/by fields

LP#1635354 Invoice close date/by fields

Acquisitions invoices have 2 new fields:

* Close Date -- This is set to the time when the ACQ user clicks the
  "Close" button in the invoice interface.

This field replaces the existing "Complete" field.  An invoice is now
considered complete if a close date value is set.

* Closed By -- This is set to the logged in staff user who performs the
  "Close" action.

As with the now-defunct 'complete' field, but new fields are cleared in the
event an invoice is reopened.

These new fields are visible in the invoice interface under the 'Show
Details' action for closed invoices.

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