Add a test for tax interaction

Problem/Motivation

There was taxes shoved into the GiftcardRedemptionPaneTest but they were breaking the tests and the existing tests weren't testing the interaction of taxes and gift cards in any useful way.

Proposed resolution

Add a tax test. Depending on tax rules gift card purchases can be taxable or not. They can be considered exempt in some circumstances.

Remaining tasks

• Document some tax rules around gift cards and when they're exempt or taxed.
• Create a test to ensure taxes are calculated correctly both when purchasing gift cards and when using them.