[Plugin] Enhanced custom validation (#6410)

* Use registry.get_plugin()

- Instead of registry.plugins.get()
- get_plugin checks registry hash
- performs registry reload if necessary

* Add PluginValidationMixin class

- Allows the entire model to be validated via plugins
- Called on model.full_clean()
- Called on model.save()

* Update Validation sample plugin

* Fix for InvenTreeTree models

* Refactor build.models

- Expose models to plugin validation

* Update stock.models

* Update more models

- common.models
- company.models

* Update more models

- label.models
- order.models
- part.models

* More model updates

* Update docs

* Fix for potential plugin edge case

- plugin slug is globally unique
- do not use get_or_create with two lookup fields
- will throw an IntegrityError if you change the name of a plugin

* Inherit DiffMixin into PluginValidationMixin

- Allows us to pass model diffs through to validation
- Plugins can validate based on what has *changed*

* Update documentation

* Add get_plugin_config helper function

* Bug fix

* Bug fix

* Update plugin hash when calling set_plugin_state

* Working on unit testing

* More unit testing

* Move get_plugin_config into registry.py

* Move extract_int into InvenTree.helpers

* Fix log formatting

* Update model definitions

- Ensure there are no changes to the migrations

* Comment out format line

* Fix access to get_plugin_config

* Fix tests for SimpleActionPlugin

* More unit test fixes

InvenTree/label/models.py

@@ -15,11 +15,11 @@ from django.urls import reverse
 from django.utils.translation import gettext_lazy as _
 
 import build.models
+import InvenTree.models
 import part.models
 import stock.models
 from InvenTree.helpers import normalize, validateFilterString
 from InvenTree.helpers_model import get_base_url
-from InvenTree.models import MetadataMixin
 from plugin.registry import registry
 
 try:
@@ -88,7 +88,7 @@ class WeasyprintLabelMixin(WeasyTemplateResponseMixin):
         self.pdf_filename = kwargs.get('filename', 'label.pdf')
 
 
-class LabelTemplate(MetadataMixin, models.Model):
+class LabelTemplate(InvenTree.models.InvenTreeMetadataModel):
     """Base class for generic, filterable labels."""
 
     class Meta: