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)

It's just a super-failsafe. If you try to create a reservation w/o a start or due date AR will still try to run all the validations, in which case start_date_before_due_date will crash the app instead of raising an exception