migration to add rebookable boolean, default false to appointments
migration to create rebookings table
appointment model validates presence of re_bookable
TODO comment about dependent destroy on ALL models
huge feature addons to appointment model. Rebookings associations, validation of re_booking must be inclusion in true false since it is a boolean field, updates to overlap query to only include active appointments - ones that are not re-bookable - in the overlap validation
rebooking model belongs to new and dead appointment