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)

This is just an AR Relation? So line 25 then builds on this, chains two more scopes, and counts however many reservations remain? (Sorry for the stupid question, but I've read this code over three times and it only clicked now)