654 blackouts

* removes equipment model column from blackouts
* simplifies model
* remove blackout helper and move blackout flashes to cart reservations
* add rake task to delete all old blackouts
Should we hold on to blackouts for a little bit, just to be safe? What about keeping them for a month or so (or even making this user-configurable, if we feel it's necessary). Either way, the timing should be both explicitly specified on the relevant views and well documented in our documentation.