Fixed #18355 -- Add ordering options to generic views.

Added new attribute and overrideable method to MultipleObjectMixin
to allow easy sorting of the queryset.

Refs #21450.
There's no low level unit test - I can add one if that's what you mean?

I think the cases are covered by my new tests (overriding the ordering) and an existing test (test_archive_view) although actually not very explicitly.