Issue #2350823 by lauriii, jhedstrom, Tim Bozeman, cilefen, davidhernandez, inqui, Cottser, xjm: Use the Classy theme in the Testing profile

Issue #2329501 by alexpott, mdrummond, davidhernandez | Cottser: Add classy.info.yml to core, set Classy as base theme for Bartik and Seven.