[1.5.x] Refactored proxy model skipping in get_default_columns()
[1.5.x] Refactored proxy model skipping in get_default_columns()

The refactoring allows custom subclasses to use different default
columns than the base model.

alx-eu/django,hasadna/django,hasadna/django,ccn-2m/django,ccn-2m/django,ccn-2m/django,imtapps/django-imt-fork,ccn-2m/django,alx-eu/django,bliti/django-nonrel-1.5,bliti/django-nonrel-1.5,hasadna/django,imtapps/django-imt-fork,imtapps/django-imt-fork,alx-eu/django,alx-eu/django,bliti/django-nonrel-1.5