This will make it easier to type the raw message in
process_new_message. The blueslip error is left here
as a precaution, but I'm fairly confident that if
RawMessage and LocalMessage both have reactions then
that codeblock will never be called.
Since we allow calling `add_messages` without checking fetch status,
it can lead to non-contiguous message history due to latest message
being added to a message list without previous messages being
fetched.
To fix it, we only allow adding new messages via message_fetch
which properly sets `anchor` to the last message in the list
before fetching and adding messages to the list.
