ref https://linear.app/tryghost/issue/ONC-274
This fix is done at the endpoint layer, which isn't ideal, but has the smallest
surface area for the change. I think we may want to move it up a layer though,
despite the extra complications. If we move the check _into_ the MembersAPI
class however, we start to run into issues with circular dependncies due to the
mess that is our dependency injection approach with the Members service
2d00d5ca0e