[FIX] payment_*: always read callback_eval as superuser

The callback_eval field has a groups parameter of
base.group_system. Without this patch everyone not part of that group
ends up with an access right error when the system attempts to read
that field.

Previously this was not a problem because all code reading
callback_eval was executed with the superuser already. New code has
been introduced however that does not do this (eg. paying with a
payment.token from the backend).

opw-741181