calculate tax after post processing all line_item types

Moved the tax calculation in `postprocess_order` to after the `postprocess"#{line_item_type}"` so that the tax can be calculated on all items that have the `[:reduce_tax_subtotals]` keys like Coupons.

This fixes the issue in the demo where tax when checking out with coupons would not calculate with the coupon  reduction in tax amount. It was working fine on the front end calculation, just not on `:create`

I originally opened an issue for the PiggybakCoupons extension, but found the issue in the Piggybak::Order model. Fixed solved the problem.