Make "continue" button after last video optional

[maramather]

Noah says that we have had two participants "fail" to complete all of the pre- baseline because they didn't click the "continue" button after the final video. To avoid this we should either find a way that the "video" task gets completed without clicking the button (though this would probably only apply in set 6, because in set 1 we do want them to click the button to progress to the next task) or we should drop the video from set 6 altogether.