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)

Under what circumstances are either start_date or due_date nil?