When media is deleted, delete associated comments too.

The actual code is just a simple for loop; there might be a better
implementation but this is a fine start.  I also extended test_delete to
check this too.

mediagoblin/user_pages/views.py

@@ -173,6 +173,10 @@ def media_confirm_delete(request, media):
         if form.confirm.data is True:
             username = media.get_uploader.username
 
+            # Delete all the associated comments
+            for comment in media.get_comments():
+                comment.delete()
+
             # Delete all files on the public storage
             delete_media_files(media)