Fundamental Rewrite of Validation System

* Moving all 'soft' validations to cart validations instead of reservation validations. This enables the cart to run most validations only one time instead of for every equipment model in the cart (available, renewable, duration, max cat count, max model count, reserver has overdue, blackout dates)
* Reservation.validateSet is no more
* Only necessary validations run when the dates are updated and when items are added/removed (doesn't check for blackout dates when adding a new item to cart, for instance)

Actually it shouldn't matter since start_date_before_due_date would trigger in the case that the due date is before Date.today and the start_date wasn't. Since in that case its not clear whether or not that reservation was intended to be made in the past or not it would be clearer to leave out not_in_past.