Add option to enable/disable sounds for specific notification types

[jmcken1]

Presently, notification sounds are either universally on or universally off. It would be very useful if we could choose which notification types have sounds and which are muted; for instance, we could choose to have sounds for mentions, but no sounds for likes and boosts. This would be handy as I don’t want to miss any mentions, but I really don’t want to hear a sound every time someone likes a post – this would become unbearable on posts that go viral and attract dozens or hundreds of likes & boosts.