Update alert group view receive channel filter

[matiasb]

We were noticing some discrepancies in alert groups counts.

From a real example:

# queryset is the alert groups initial queryset
>>> qs = queryset.filter(channel_filter__alert_receive_channel=arc)
>>> qs.filter(resolved_at__isnull=False).count()
1318
>>> qs = queryset.filter(channel=arc)
>>> qs.filter(resolved_at__isnull=False).count()
1356
>>> qs.filter(resolved_at__isnull=False, channel_filter__isnull=True).count()
38