fix the error happens in the case of a new review is not saved
improve the descriptions in sessions_controller_spec a little
revise the spec of VideosController show action to pass the validation of uniqueness of user_id in scope of video_id
modify UsersController and its spec a little bit
finish function: users can see the queue
finish the function: users can add videos to their own queue
finish function: users can delete items from their own queue