Fixed #21414 -- Removed usage of RelatedObject and deprecated it.

The django.db.models.related module is now deprecated. The functionality
was moved to ForeignObjectRel which is accessible from field.rel.
RelatedObject was accessible from field.related.
I don't have strong feelings about this. One possibility is to remove this, and if we get complaints about the removal during alpha or beta we can reintroduce it back. Another approach is to check how hard it is to write code that works both in 1.7 and 1.8 for those cases that use RelatedObject.