Issue #2581443 by alexpott, lauriii, Cottser: Make Classy extend from the new Stable base theme

...lib/Drupal/Core/Theme/MissingThemeDependencyException.php

core/lib/Drupal/Core/Theme/ThemeInitialization.php

core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php

...dules/config/src/Tests/ConfigImportInstallProfileTest.php

...les/system/src/Tests/Update/UpdatePathRC1TestBaseTest.php

core/modules/system/system.install

core/themes/classy/classy.info.yml