performance: prefetch categories in listen page (190 -> 12 SQL queries)

panikweb/views.py

@@ -482,7 +482,7 @@ class Listen(TemplateView):
     def get_context_data(self, **kwargs):
         context = super(Listen, self).get_context_data(**kwargs)
         context['sectionName'] = "Listen"
-        context['episodes'] = Episode.objects.filter(
+        context['episodes'] = Episode.objects.prefetch_related('emission__categories').filter(
                 soundfile__podcastable=True, soundfile__fragment=False) \
                 .select_related().extra(select={
                     'first_diffusion': 'emissions_diffusion.datetime', },