SI: Tax calculation performed in wrong order

[WillStrohl]

In some instances, the tax calculation step is performed in the wrong order, causing the customer to be over-charged for taxes.

Steps to Reproduce:

1. Clean installation of DNN 07.04.02 with clean installation of Hotcakes 02.00.07 installed. (Also able to be reproduced in Hotcakes 03.01.00.)
2. Create a shipping method: flat rate per order of $5.95.
3. Create a tax schedule and set at 6% each for subtotal and shipping for California.
4. Create a product for $65 that's shippable and assigned to the California tax schedule.
5. Create a promotion to discount the entire order by 10% if a specific coupon code is entered.
6. (In a new web browser...)
7. Add a quantity of 4 the new product to the cart.
8. In the cart, enter the coupon code for the discount.
9. In the checkout, enter an address matching the shipping zone.

Expected Behavior:
The tax is $15.96 without a coupon, and it's $14.40 with the coupon since the taxable amount for the line item is lower due to the 10% discount.

... without the coupon
6% of $260 = $15.60
6% of $5.95 = $00.36
Grand Total = $15.96

... with the coupon
6% of $234 = $14.04
6% of $5.95 = $00.36
Grand Total = $14.40

Actual Behavior:
The tax is $15.96 regardless of the coupon.

[WillStrohl]

In the case of VAT, it appears that taxes are applied differently:

https://informi.co.uk/articles/how-vat-affected-discounts

The tax is calculated on the price before discount.

[WillStrohl]

Merged into dev and verified during testing.