Add hook for BaseModelAdmin.raw_id_fields

[Ivan Dominic Baguio]

BaseModelAdmin.raw_id_fields is a very feature in django admin that prevents page timing out or crashing when a related field's table has too many records for the default django admin dropdown widget. Alternatively, autocomplete_fields has the same objective, but using a different widget.

Currently, there exists a method hook BaseModelAdmin.get_autocomplete_fields(request) which can be overridden by child admin classes for customized logic. But there is no get_raw_id_fields hook that allows us to add some logic for raw_id_fields. It would be useful to have this hook also included in the admin. Right now we are working around this by adding our custom logic to populate the raw_id_fields in the __init__ method instead.

We need this code to be added to the BaseAdmin class.

def get_raw_id_fields(self, request):
    """
    Return a list of ForeignKey and/or ManyToMany fields which should use
    a raw id widget.
    """
    return self.raw_id_fields

--
The specific use case we have for this is we are writing a custom admin class that automatically sets all of the related fields (FK, O2O, M2M) of a model to raw_id_fields or autocomplete_fields so devs doesn't have to manually configure the admin every time a new field is added / removed to the model.

[Ivan Dominic Baguio]

Patch that implements the requested feature

[Mariusz Felisiak]

Duplicate of #17881.