Issue #1189806 by mortendk, lauriii, jussil, Dragan Eror, cluke009, DickJohnson, rteijeiro, tlattimore: Convert aggregator-item.html.twig to HTML5

Issue #2349759 by davidhernandez, mortendk, rteijeiro, harish86, Sumit kumar, saki007ster, Mukeysh, brahmjeet789, nitishchopra, falkendk, Maninder, Vidushi Mehta, Ashish sandil, jitendra verma, crowdcg, sivaji@knackforge.com, dernetzjaeger: Copy system templates to Classy.