total_price for orders (#4447)

* Adds unit test for counting queries on PurchaseOrderList API endpoint

- We will work to make this queryset more efficient

* PEP fixes

* Add 'total_price' fields to SalesOrder and PurchaseOrder models

* PurchaseOrder list API now has constant query count

* Data migration for updating existing PurchaseOrder and SalesOrder instances

- Calculate total_price for any existing order
- Will fail if exchange rates are not available

* Add total_price_currency to API serializers

* Render total_price in SalesOrder table

* Add ability to filter both lists by total_price field

* Update total_price whenever an order is updated

* Update total price whenever a lineitem is saved or deleted

* Add query-counting unit test for SalesOrder list API

* Calling refresh_from_db inside a save() method is *not* a good idea

InvenTree/part/test_pricing.py

@@ -2,7 +2,6 @@
 
 from django.core.exceptions import ObjectDoesNotExist
 
-from djmoney.contrib.exchange.models import ExchangeBackend, Rate
 from djmoney.money import Money
 
 import common.models
@@ -18,31 +17,6 @@ from InvenTree.status_codes import PurchaseOrderStatus
 class PartPricingTests(InvenTreeTestCase):
     """Unit tests for part pricing calculations"""
 
-    def generate_exchange_rates(self):
-        """Generate some exchange rates to work with"""
-
-        rates = {
-            'AUD': 1.5,
-            'CAD': 1.7,
-            'GBP': 0.9,
-            'USD': 1.0,
-        }
-
-        # Create a dummy backend
-        ExchangeBackend.objects.create(
-            name='InvenTreeExchange',
-            base_currency='USD',
-        )
-
-        backend = ExchangeBackend.objects.get(name='InvenTreeExchange')
-
-        for currency, rate in rates.items():
-            Rate.objects.create(
-                currency=currency,
-                value=rate,
-                backend=backend,
-            )
-
     def setUp(self):
         """Setup routines"""