Improvements to greeting feature

• Detect whether someone is rejoining (if their previous membership event was to leave) and give a shorter welcome back message.
• Detect whether the room is an upgrade of a previous one, and detect if anyone joining for the first time is in the previous room, and give a shorter "welcome to the new room" message. It might be helpful to batch these, to reduce spam.
• Make a guess at whether a new user is a guest (if their account name is all numbers on flip.earth, it is pretty likely) and send a different message. This could be focused on what it means to be a guest and how to create a new account.