[5.3] Allows to customize broadcast channel name for broadcasting notification channel

[kduma]

Allows to customize broadcast channel name for broadcasting notification channel.

To set channel name, place this method in your notifiable class:

    public function broadcastNotificationChannelName($notification)
    {
        return 'notifications.'.$this->id;
    }

I've made this PR because I wanted to get rid of long channel name like App.Models.Users.User.*, and hide class structures from an end user. It would be also great if anyone would like to encrypt user id using something like HashIds:

    public function broadcastNotificationChannelName($notification)
    {
        return 'App.User.' . Hashids::encrypt($this->id);
    }