Set or clear the visitor's reaction to a message

Called by the embedded chat surfaces with the visitor's chat-session credentials, the only credential this route accepts. The visitor holds at most one reaction per message; a new emoji replaces it and emoji: null takes it back. Internal notes and operator-only conversations cannot be reacted to. Responds with only the message id and its reactions as a visitor may see them — which emojis, and whether each came from the visitor or from the team.