updateChatMessage(id: string,body: string,requestingUserId: string,canModerate: boolean): ChatMessage
Replaces a message's body on a chat_edit, and claims any image the new body
links to. canModerate lets a moderator edit anyone's message. The flag
mentionsEveryone is set again from the new body and the author's role, not
the editor's: a moderator's edit can't make a user's message reach everyone.
The caller broadcasts the result.