Rules: Notification of Payment for Invoice Approvers

Tomorrow.io has requested an email notification to be sent to Invoice Approvers when invoice they have approved have been paid. I believe that if we do create this email notification, that invoice approvers should have the ability to turn this notification on/off in the event that they are annoyed by the emails.

It might also make sense to include a Paid Invoice tab in the UI for invoice approvers where they can review ONLY the invoice payment status of the invoices they have approved.