Fixed #19044 -- Made DeletionMixin interpolate its success_url.

Thanks to nxvl and slurms for the initial patch, ptone for the review
and timo for the documentation tweaks.

docs/ref/class-based-views/mixins-editing.txt

@@ -201,6 +201,13 @@ ProcessFormView
         The url to redirect to when the nominated object has been
         successfully deleted.
 
+        .. versionadded:: 1.6
+
+        ``success_url`` may contain dictionary string formatting, which
+        will be interpolated against the object's field attributes. For
+        example, you could use ``success_url="/parent/%(parent_id)s/"`` to
+        redirect to a URL composed out of the ``parent_id`` field on a model.
+
     .. method:: get_success_url(obj)
 
         Returns the url to redirect to when the nominated object has been