Add a site setting to apply Discount Amount to the net price instead of the gross price when Tax Part is used (OCC-424)

[infofromca]

Describe the bug

A clear and concise description of what the problem is.

To Reproduce

Steps to reproduce the behavior:

1. Go to 'Test Discounted Product'

2. see screenshot

3. See error on shopping cart

Expected behavior

5-2=3*(1 +0.13)=3.39

Screenshots

Jira issue

[sarahelsaig]

This is not a bug, it works as designed. Discount amount applies to the gross price:

(5*1.13)-2

instead of

(5-2) * 1.13

The current behavior is the normal expectation for shops that display the gross price. I realize this is opinionated, biased against countries where only net price is displayed. But I know of existing users who already rely on the current arrangement so it will always be the default. Yet a site setting should be added to optionally toggle to the logic you requested.

I've updated the issue accordingly.