Extend functionality of custom validation plugins (#4391)

* Pass "Part" instance to plugins when calling validate_serial_number

* Pass part instance through when validating IPN

* Improve custom part name validation

- Pass the Part instance through to the plugins
- Validation is performed at the model instance level
- Updates to sample plugin code

* Pass StockItem through when validating batch code

* Pass Part instance through when calling validate_serial_number

* Bug fix

* Update unit tests

* Unit test fixes

* Fixes for unit tests

* More unit test fixes

* More unit tests

* Furrther unit test fixes

* Simplify custom batch code validation

* Further improvements to unit tests

* Further unit test

InvenTree/part/test_pricing.py

@@ -215,7 +215,7 @@ class PartPricingTests(InvenTreeTestCase):
         # Create a part
         p = part.models.Part.objects.create(
             name='Test part for pricing',
-            description='hello world',
+            description='hello world, this is a part description',
         )
 
         # Create some stock items