Optimized getting threads for tag.

boards/models.py

@@ -68,7 +68,7 b' class PostManager(models.Manager):'
     def get_threads(self, tag=None, page=ALL_PAGES,
                     order_by='-last_edit_time'):
         if tag:
-            threads = self.filter(thread=None, tags=tag)
+            threads = tag.threads
 
             # TODO This needs to be uncommented when 'all tags' view won't
             # use this method to get threads for tag