Returned by chat-service's query, which joins each message to its author and
to the message it replies to. reply_to_id is kept when the message it
points to is deleted, while reply_to_author and reply_to_body then read
null.
id: string
user_id: string
body: string
created_at: string
updated_at: string | null
author_username: string
author_avatar_mime: string | null
reply_to_id: string | null
reply_to_author: string | null
reply_to_body: string | null
Cut to chat-service's REPLY_SNIPPET_LEN characters.
mentions_everyone: number
1 or 0.