Make it possible to override object retrieval in `compare_view` method.

[NikAzanov]

In the BaseCompareVersionAdmin.compare_view method, the current implementation of object retrieval uses get_object_or_404 for the model. This method uses _default_manager, which makes it impossible to work with logically deleted objects. It would be nice to have a separate method for getting an object or something like that, so it can be easily overridden and work with the needed queryset.

[jedie]

Pull requests are welcome. Plesse with a complete test ;)