"Start next puzzle nobody has seen" for logged in users only

[hftf]

Will be disabled if at least one person in the room is logged out, with message "sign up or log in" instead.

However it will still be nearly impossible to prevent people from playing puzzles while logged out (or, if we require auth, from using sock puppets). A middle solution might be making it easy to stay logged in, somewhat like Protobowl (keep in mind they don't have a real accounts feature yet).