Fixed #24215 -- Refactored lazy model operations

This adds a new method, Apps.lazy_model_operation(), and a helper function,
lazy_related_operation(), which together supersede add_lazy_relation() and
make lazy model operations the responsibility of the App registry. This
system no longer uses the class_prepared signal.

docs/releases/1.9.txt

@@ -372,6 +372,8 @@ Miscellaneous
   :class:`~django.forms.SelectDateWidget` in ``django.forms.widgets``
   (or simply ``django.forms``) instead.
 
+* Private API ``django.db.models.fields.add_lazy_relation()`` is deprecated.
+
 .. removed-features-1.9:
 
 Features removed in 1.9