fix(medusa): calculates correct taxes and totals on order with gift cards (#1807)

**What**
Since the release of the Tax API the line item totals calculations on orders with gift cards have been wrong. To understand the bug consider the below order:

Region:
- tax_rate: 25%
- gift_cards_taxable: true

Order:
- applied gift card: 1000
- items: 
  - A: unit_price: 1000
  - B: unit_price: 500
- Subtotal: 1500

**Previous calculation method**
1. Determine how much of the gift card is used for each item using `item_total / subtotal * gift_card_amount`:
  - Item A: 1000/1500 * 1000 = 666.67
  - Item B: 500/1500 * 1000 = 333.33
2. Calculate line item totals including taxes using `(unit_price - gift_card) * (1 + tax_rate)`
  - Item A: 1000 - 666.67 = 333.33; vat amount -> 83.33
  - Item B: 500 - 333.33 = 166.67; vat amount -> 41.67
3. Add up the line item totals: order subtotal = 500; vat amount = 125; total = 625

This is all correct at the totals level; but at the line item level we should still use the "original prices" i.e. the line item total for item a should be (1000 * 1.25) = 1250 with a tax amount of 250. 

**New calculation method**
1. Use default totals calculations
  - Item A: subtotal: 1000, tax_total: 250, total: 1250
  - Item B: subtotal: 500, tax_total: 125, total: 625
2. Add up the line item totals: subtotal: 1500, tax_total: 375, total: 1875
3. Reduce total with gift card: subtotal: 1500 - 1000 = 500, tax_total: 375 - 250 = 125, total = 625

Totals can now be forwarded correctly to accounting plugins.

Fixes CORE-310.

integration-tests/plugins/__tests__/medusa-plugin-sendgrid/__snapshots__/index.js.snap

@@ -753,6 +753,7 @@ Object {
   "external_id": null,
   "fulfillment_status": "canceled",
   "fulfillments": Array [],
+  "gift_card_tax_total": 0,
   "gift_card_total": "0.00 USD",
   "gift_card_transactions": Array [],
   "gift_cards": Array [],
@@ -975,6 +976,7 @@ Object {
   "external_id": null,
   "fulfillment_status": "fulfilled",
   "fulfillments": Array [],
+  "gift_card_tax_total": 0,
   "gift_card_total": "0.00 USD",
   "gift_card_transactions": Array [],
   "gift_cards": Array [],
@@ -1244,6 +1246,7 @@ Object {
         "updated_at": Any<Date>,
       },
     ],
+    "gift_card_tax_total": 0,
     "gift_card_total": 0,
     "gift_card_transactions": Array [],
     "gift_cards": Array [],