refactor complex method available? in ReservationValidations

please add checks to filter so that it only returns approval_status == 'auto' or 'approved'. your new code already filters that out via the not_returned scope, but I think it would be good to filter here also in case anyone ever calls reserved_in_date_range by itself. another thing I should have considered before, in #297, is setting up a scope for just approval_status == 'auto' or 'approved', and then chaining it onto the other scopes like we're already doing with .recent for example.