Add the ability to add custom ringtones to people

Basically, allow users to customise ringtones for specific users (either with or without nitro)

i.e., Significant Other#0001 has Rain on Brick - Katana Zero playing when they call (hell, even message!)

Childhood Friend#6969 plays Fireflies - Khai Dreams when they call

This was the last thing I really liked about Skype and I'd love to see this on my fave IM, Discord!