Ticket 16311

- Provide a hook into ``admin.filter.RelatedFieldListFilter`` to change the related objects to display.
- Implements ``RelatedOnlyFieldListFilter``which use that hook to limit the related objects to the ones found into the relation.
This could be a huge `__in` query would is inefficient. I tried something like:
```
limit_choices_to = {
    '%s__isnull' % model_admin.model._meta.get_field(field.name).related_query_name(): False,
}
```
but this can result in duplicates. Perhaps it would be viable if we resolved https://code.djangoproject.com/ticket/11707 first.