Respect desktop's Do Not Disturb mode for desktop notifications

The Discord desktop app and website don't fully respect Do Not Disturb mode, at least on Linux. Sending the notification is fine (as the notifications get inhibited), but Discord will always make a notification sound, regardless of whether notifications are inhibited or not.

Ideally, Discord would check on dbus whether notifications are inhibited (`org.freedesktop.Notifications.Inhibited`) and would set its own Do Not Disturb mode to match that. If that's not acceptable, not making the notification sound if notifications are inhibited would be a reasonable secondary option.