notification_sounds: Add sounds created by @stwime.

These are all good notification sounds that may fit taste of
different users.

docs/THIRDPARTY

@@ -134,6 +134,10 @@ Files: zerver/decorator.py zerver/management/commands/runtornado.py scripts/setu
 Copyright: Django Software Foundation and individual contributors
 License: BSD-3-Clause
 
+Files: static/audio/notification_sounds/*
+Copyright: 2021 Stefan Mertens
+License: CC0-1.0
+
 License: Apache-2.0
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.