Also, add a new notification sound, "ding". It comes from
https://freesound.org, where the original Zulip notification sound comes
from as well. In the future, new sounds can be added by adding audio
files to the `static/audio/notification_sounds` directory.
Tweaked significantly by tabbott:
* Avoided removing static/audio/zulip.ogg, because that file is
checked for by old versions of the desktop app.
* Added a views check for the sound being valid + tests.
* Added additional tests.
* Restructured the test_events test to be cleaner.
* Removed check_bool_or_string.
* Increased max length of notification_sound.
* Provide available_notification_sounds in events data set if global
notifications settings are requested.
Fixes#8051.
Marco Burstein