Deliberately not a bulk PUT of the whole grid. A client that posts every pair it
knows about would silently revert any preference recorded by a channel it has not
been taught about yet — an SMS STOP, or an admin acting on a phone call — and the
person would be re-enrolled in something they refused, by a UI that never showed it.
One decision about one channel/category pair.
Deliberately not a bulk PUT of the whole grid. A client that posts every pair it knows about would silently revert any preference recorded by a channel it has not been taught about yet — an SMS STOP, or an admin acting on a phone call — and the person would be re-enrolled in something they refused, by a UI that never showed it.