(Credit: Dominic M. Mercier) Opera Philadelphia will kick off its O19 Festival with the world premiere of Venables and Huffman’s opera “Denis & Katya.” The opera, co-commissioned and co-produced with Music Theatre Wales and Opéra Orchestre National Montpellier, was inspired by the “true story of two 15-year-old runaways who became social media sensations when they live streamed an armed stand-off {…}