Closes #19924: Record model features on ObjectType (#19939)

* Convert ObjectType to a concrete child model of ContentType

* Add public flag to ObjectType

* Catch post_migrate signal to update ObjectTypes

* Reference ObjectType records instead of registry for feature support

* Automatically create ObjectTypes

* Introduce has_feature() utility function

* ObjectTypeManager should not inherit from ContentTypeManager

* Misc cleanup

* Don't populate ObjectTypes during migration

* Don't automatically create ObjectTypes when a ContentType is created

* Fix test

* Extend has_feature() to accept a model or OT/CT

* Misc cleanup

* Deprecate get_for_id() on ObjectTypeManager

* Rename contenttypes.py to object_types.py

* Add index to features ArrayField

* Keep FK & M2M fields pointing to ContentType

* Add get_for_models() to ObjectTypeManager

* Add tests for manager methods & utility functions

* Fix migrations for M2M relations to ObjectType

* model_is_public() should return False for non-core & non-plugin models

* Order ObjectType by app_label & model name

* Resolve migrations conflict

netbox/extras/models/customfields.py

@@ -72,7 +72,7 @@ class CustomFieldManager(models.Manager.from_queryset(RestrictedQuerySet)):
 
 class CustomField(CloningMixin, ExportTemplatesMixin, ChangeLoggedModel):
     object_types = models.ManyToManyField(
-        to='core.ObjectType',
+        to='contenttypes.ContentType',
         related_name='custom_fields',
         help_text=_('The object(s) to which this field applies.')
     )
@@ -84,7 +84,7 @@ class CustomField(CloningMixin, ExportTemplatesMixin, ChangeLoggedModel):
         help_text=_('The type of data this custom field holds')
     )
     related_object_type = models.ForeignKey(
-        to='core.ObjectType',
+        to='contenttypes.ContentType',
         on_delete=models.PROTECT,
         blank=True,
         null=True,