Fix a regression for custom feeds (e.g. Soundcloud)

Refactoring introduced a bug where new episodes were
never found on custom feeds such as Soundcloud.

src/gpodder/model.py

@@ -176,6 +176,10 @@ class PodcastChannel(PodcastModelObject):
         self.save()
 
         guids = [episode.guid for episode in self.get_all_episodes()]
+
+        # Insert newly-found episodes into the database
+        custom_feed.get_new_episodes(self, guids)
+
         self.save()
 
         self.db.purge(max_episodes, self.id)