Closes #13427: Public model registration (#14152)

* Initial work on #13427

* Clarify documentation

* Reference public models registry when populating models for ConfigTemplate context

netbox/users/models.py

@@ -99,6 +99,8 @@ class UserConfig(models.Model):
         default=dict
     )
 
+    _netbox_private = True
+
     class Meta:
         ordering = ['user']
         verbose_name = _('user preferences')