clean up a bunch of prize stuff

* delete some controllers\views that are already not linked to and shouldn’t be used
* stop sending an outdated email on the completion by the 15th student
* stop calculating/setting User.prize_earned
* don't link to prize page unless you have already redeemed prizes
* remove unused partials

Kept:
* prize section of admin stats report (http://studio.code.org/admin/stats)
* page for teachers who have already earned prizes to see them (http://studio.code.org/redeemprizes)
you could remove the PrizeProvider, Prize, etc. for cannot :read here, right?