* 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
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