Observer::notify('user_after_XXX') change

All three notifiers **shouldn't** use `$user->name` as passed variable, since `$user->name` cannot **uniquely** identify user. (`$user->username` does )

More meaningful variable passed to observer will be `$user->id` - which 
can uniquely identify user's record in DB.

**Added** second parameter `$user->id` and *leaved* `$user->name` for backwards compatibility.