fix issue #363

Unreasonable on admin user online stats and forum user online stats of ACP.
Instead of this, you can make "admin_sessions" variable above render_to call:

```
sessions_expiration = timezone.now() - timedelta(seconds=settings.online_counting_frequency)
admin_sessions = Session.objects.filter(user__isnull=False).filter(admin=1)
admin_sessions = admin_sessions.filter(last__gte=sessions_expiration)
admin_sessions = admin_sessions.order_by('user__username_slug').select_related('user')
```

You can do same for online view.