Set or clear the signed-in operator's reaction to a message

Each person holds at most one emoji reaction per message: a new emoji replaces the operator's previous one, and emoji: null takes it back. Any message of the operator's organization may be reacted to, internal notes included; reactions on notes and operator-only conversations never reach the visitor. Emits MESSAGE_UPDATED_EVENT to the dashboard and, for a message the visitor can see, an id-only MESSAGE_UPDATED_EVENT on the visitor's channel.