Issue #1751606 follow-up by sun, swentel: Various fixes for content creation page.

core/includes/form.inc

core/includes/theme.inc

core/misc/dropbutton/dropbutton.base.css

core/modules/node/lib/Drupal/node/NodeFormController.php

...odules/node/lib/Drupal/node/Tests/NodeFormButtonsTest.php

core/modules/node/node.admin.css

core/modules/node/node.admin.inc

...es/system/lib/Drupal/system/Tests/Theme/FunctionsTest.php

core/modules/system/system.module

core/themes/seven/style.css