After T5369, we should have infrastructure for playing sounds without duplicating them across windows.

• Make or find at least one permissively licensed sound which no one hates too much.
• Add a settings panel to let users disable sounds (Settings → Sound, provide a "never play any sounds" option at a minimum).
• Add the sound for chat in an open thread.

Things we might want to pursue eventually, but which are not in scope for this task:

• Special sound for a username mention (see T7580).
• Several sound options.
• Selectable sounds per thread.
• Mute sounds for a given thread.
• Sounds for other notifications?