Restore self.object

Though this was removed in fd1eda4
it seems to cause issues with prefetch_related_objects() in certain
cases, though it shows up inconsistently in different environments.

promgen/views.py

@@ -780,6 +780,8 @@ def form_invalid(self, **kwargs):
         return self.render_to_response(self.get_context_data(**kwargs))
 
     def post(self, request, *args, **kwargs):
+        self.object = self.get_object()
+
         # Save a copy of our forms into a context var that we can use
         # to re-render our form properly in case of errors
         context = {}