Pricing bug fix (#4422)

* Control when a new PartPricing object can be created

- Prevent this when calling from an on_delete signal
- There is an edge case where deleting a part triggers a series of on_delete signals inside an atomic transaction
- When the new PartPricing object is created,

* Add unit testing:

- Ensure PartPricing gets created when a new StockItem is added
- Part.delete() works without error
- PartPricing instances are deleted also

* style fixes

InvenTree/company/migrations/0026_auto_20201110_1011.py

@@ -23,7 +23,7 @@ def migrate_currencies(apps, schema_editor):
     for the SupplierPriceBreak model, to a new django-money compatible currency.
     """
 
-    logger.info("Updating currency references for SupplierPriceBreak model...")
+    logger.debug("Updating currency references for SupplierPriceBreak model...")
 
     # A list of available currency codes
     currency_codes = CURRENCIES.keys()