actions: Mark all messages as read when user unsubscribes from stream.

This fixes a bug where, when a user is unsubscribed from a stream,
they might have unread messages on that stream leak.  While it might
seem to be a minor problem, it can cause significant problems for
computing the `unread_msgs` data structures, since it means we need to
add an extra filter for whether the user is still subscribed, either
in the backend or in the UI.

Fixes #7095.

tools/test-queue-worker-reload

@@ -19,6 +19,7 @@ TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
 successful_worker_launches = [
     'launching queue worker thread error_reports',
     'launching queue worker thread user_presence',
+    'launching queue worker thread deferred_work',
     'launching queue worker thread digest_emails',
     'launching queue worker thread slow_queries',
     'launching queue worker thread missedmessage_mobile_notifications',