Add an option to adjust volume for discord notifications

I personally find the notifications for Discord a bit loud and would rather tone them down to a preferable volume of my choosing. While I can lower the volume of discord itself, it would affect user volumes as well which would have all to be adjusted, and is awkward to deal with.

This idea would be implemented as an adjustable volume bar, the same way you would adjust input and output volume in Voice and Video settings, but would be located within the Notifications tab. Adjusting the volume bar would affect all types of alerts, including pings, voice notifs, stream notifs, etc.