Issue #2218039 by sun, dawehner: Render the maintenance/install page like any other HTML page.

core/authorize.php

core/includes/batch.inc

core/includes/errors.inc

core/includes/install.core.inc

core/includes/theme.inc

core/includes/theme.maintenance.inc

core/includes/update.inc

core/lib/Drupal/Core/Controller/ExceptionController.php

...Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php

core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php

...lib/Drupal/system/Tests/Common/RenderElementTypesTest.php

core/modules/system/templates/install-page.html.twig

core/modules/system/templates/maintenance-page.html.twig

core/themes/bartik/bartik.theme

core/themes/bartik/templates/maintenance-page.html.twig

core/themes/seven/seven.theme

core/themes/seven/templates/install-page.html.twig

core/themes/seven/templates/maintenance-page.html.twig

core/themes/seven/templates/page.html.twig

core/update.php