Show system notifications only when the user is subscribed or targeted by a notification

[Donorhan]

At the moment, every time you receive a notification you get a system notification even if you are not subscribed to a channel.

If the notification concerns a zone, it should be checked that the user is subscribed to this channel before generating a system notification.

The bug/verification to do is probably here: https://github.com/l3mpire/lemverse/blob/master/core/client/lemverse.js#L114