Configuration for "peekable room" in single room mode

On Matrix.org it can take a good while (1min+) at times to just get the room state (visibility=world_readable)

So, if we add a configuration along side single room mode (when room is specified), whether the room is peekable or not, we can skip checking for it dynamically and simply fetch messages.

This should speed up the time its required before the timeline can be rendered for the user.