lp1494427-fix refund error

lp1494427-fix refund error

We inadvertently removed staff.patron.bills.handle_refund.confirm_message with
some of the negative balance work. This branch restores it so that staff can
perform refund operations again.

Test case: Find a negative balance transaction and, from the patron's bills
record, select the transaction and select the option to Refund. Pre-patch, this
action will generate an error message. Post-patch, this action should generate
a prompt asking the user to confirm the action.

Signed-off-by: Kathy Lussier <klussier@masslnc.org>