Responsible-for-billing party field on client profiles

Invoice edits for responsible party

Add a "responsible for billing" party field within each client's profile so that invoices can be sent to a third party while the treated client record stays accurate. This is needed for cases like worker’s comp evaluations and independent educational evaluations, where the client is not responsible for billing. SimplePractice supports setting a "responsible for billing" party within each client's profile for this purpose.