purge_queue: Allow purging Tornado-consumed queues.

These are the exceptions to the rule that our queues correspond to
queue-processor workers.

Purging `notify_tornado` in particular is a useful workaround right
now for some error spew in the dev environment.

zerver/management/commands/purge_queue.py

@@ -31,7 +31,8 @@ class Command(BaseCommand):
             raise CommandError("Missing queue_name argument!")
         else:
             queue_name = options['queue_name']
-            if queue_name not in get_active_worker_queues():
+            if queue_name not in ['notify_tornado', 'tornado_return',
+                                  ] + get_active_worker_queues():
                 raise CommandError("Unknown queue %s" % (queue_name,))
 
             print("Purging queue %s" % (queue_name,))