Return top tags for a source.

def generate_additional_context(self, matching_datasets):
        """Return top tags for a source."""
        top_tags = Tag.objects.filter(
            dataset__in=matching_datasets
        ).annotate(
            tag_count=Count('word')
        ).order_by('-tag_count')[:3]

        return {
            'top_tags': top_tags
        }