[finishes #73148102]: disable teacher prizes [DO NOT MERGE UNTIL 6/15/2014]

The teacher prize promotion for the 2013-14 academic year is ending on June 15th. These changes change the dashboard in the following ways:

(1) Claiming a prize code is disabled in the prize_providers_controller
(2) An email message is no longer sent when you would have become eligible to claim a $250 bonus prize code.
(3) The email message that is sent when you would have become eligible to claim a $750 prize code is still sent, but it is a different email that explains that the promotion has ended with links to the code.org site.
(4) Visiting the learn.code.org/redeemprizes page will explain that the promotion has ended and code are no longer available.
(5) Visiting that same page if you have already claimed one or more codes will result in a slightly different message that reminds you to redeem the codes you have claimed soon (they will expire on the donorschoose website this summer)

What has not changed:
(1) the accounts will still be marked with teacher_prize_earned and teacher_bonus_prize_earned as their students achieve the thresholds
(2) the Prizes link will still appear on the user drop-down menu in the top-right of the screen.