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.
To replace the previous subscribe/subscribe buttons, we add a button
to visit channel settings (if not subscribed) or manage channel
settings (if subscribed) to trailing bookends.
Fixes#32125.
