Fix IPN bug caused by manual invoicing (Not tested in practice!)

Add handling for txn_type "invoice_payment" IPN messages.

NOTE: I didn't figure out a handy way to test this in pracitce. So suggesting to deploy and see that errors stop coming to Airbrake (and that other actions continue normally.)

But only few lines, so if someone else too takes a careful look, we should be able to minize the risk of breaking the whole IPN system due to this fix.