add order.total_tax method to encapsulate the included and additional tax

use order.total tax in google analytics

Fixes #4724
Fixes #4721
Dont mess unrelated records on CreateItemAdjustments#destroy
session[:order_id] => cookies.signed[:order_id]

Now we are using a signed cookie to store the order id on a guests
browser client.  This allows customers who close their browser to
continue their shopping when they visit again.

Fixes #4319
Improve specs for CreateItemAdjustments#destroy
Always pass currency to Spree::Money for multi-currency.
Need ability to pass currency option for multi-currency.