Disable deletion if model.PROTECTED fk linked

```
File "material/frontend/views/delete.py", line 47, in get_context_data
    kwargs.setdefault("deleted_objects", dict(self._get_deleted_objects()))
  File "material/frontend/views/delete.py", line 39, in _get_deleted_objects
    collector.collect([self.object])
  File "django/db/models/deletion.py", line 302, in collect
    raise ProtectedError(
```