createChatMessage(userId: string,body: string,replyToId?: string): ChatMessage
Stores a message from userId on a chat_send, and claims the pasted images
it links to. A replyToId naming no message is dropped, and the message is
posted as no reply. The message mentionsEveryone when it says @everyone
and userId may. The caller broadcasts the result, and notifies everyone
when it is flagged.