Fixed #18676 -- Allow fast-path deletion of objects

Objects can be fast-path deleted if there are no signals, and there are
no further cascades. If fast-path is taken, the objects do not need to
be loaded into memory before deletion.

Thanks to Jeremy Dunck, Simon Charette and Alex Gaynor for reviewing
the patch.

tests/regressiontests/admin_util/models.py

@@ -39,3 +39,6 @@ class Guest(models.Model):
 
     class Meta:
         verbose_name = "awesome guest"
+
+class EventGuide(models.Model):
+    event = models.ForeignKey(Event, on_delete=models.DO_NOTHING)