Indicate when you can't send to a room

[ulyssa]

Some rooms require a power level greater than what most members have to be able to send messages to it. There is a Room::can_user_send_message that takes care of doing the power-level checks. This could be used in refresh_rooms to take care of periodically updating a readonly: bool field on each RoomInfo object, which can then be used to disable the message bar when the user is not allowed to send.