Add support for custom billing invoice email address

Fixes #782

data/model/user.py

@@ -125,7 +125,13 @@ def change_username(user_id, new_username):
     return user
 
 
-def change_invoice_email(user, invoice_email):
+def change_invoice_email_address(user, invoice_email_address):
+  # Note: We null out the address if it is an empty string.
+  user.invoice_email_address = invoice_email_address or None
+  user.save()
+
+
+def change_send_invoice_email(user, invoice_email):
   user.invoice_email = invoice_email
   user.save()