add main_sound property, to get the main sound of an episode (#1079)

panikdb/emissions/models.py

@@ -181,6 +181,12 @@ class Episode(models.Model):
     def has_sound(self):
         return (self.soundfile_set.count() > 0)
 
+    def main_sound(self, podcastable=True):
+        t = self.soundfile_set.exclude(podcastable=False).exclude(fragment=True)
+        if t:
+            return t[0]
+        return None
+
 
 class Diffusion(models.Model):
     episode = models.ForeignKey('Episode', verbose_name=u'Episode')