Return dictionary of source section slugs, name, and counts.

def generate_sections(self):
        """Return dictionary of source section slugs, name, and counts."""
        sources = Dataset.objects.values(
            'source', 'source_slug'
        ).annotate(source_count=Count('source_slug'))

        return sorted([
            {
                'slug': source['source_slug'],
                'name': source['source'],
                'count': source['source_count']
            }
            for source in sources
        ], key=lambda k: k['count'], reverse=True)