keep track of the teasers shown to the user

since we show these pseudorandomly, it would be nice not to repeat the ones you've already seen. 

keep a running list in the browser somewhere (cookie, localstorage) and draw another teaser if you've seen it already. 

have some fallback case if the user has seen all of the teasers. maybe a congrats dialog or prompt if they want to start over and reset the running list of shown teasers. 