Issue #2066145 by mtift: Convert node_admin_theme() to the configuration system.

core/modules/book/book.module

core/modules/node/config/node.settings.yml

core/modules/node/config/schema/node.schema.yml

core/modules/node/node.install

core/modules/node/node.module

...les/overlay/lib/Drupal/overlay/Tests/OverlayCloseTest.php

.../shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php

...dules/system/lib/Drupal/system/Tests/System/ThemeTest.php

core/modules/translation/translation.module

core/profiles/standard/standard.install