Issue #2014821 by amateescu, swentel: Introduce form modes UI and their configuration entity.

core/includes/entity.inc

core/lib/Drupal/Core/Entity/EntityFormController.php

core/lib/Drupal/Core/Entity/EntityFormControllerNG.php

...lib/Drupal/custom_block/CustomBlockTypeListController.php

core/modules/comment/comment.module

core/modules/contact/contact.module

...les/contact/lib/Drupal/contact/CategoryListController.php

.../contact/lib/Drupal/contact/Tests/ContactSitewideTest.php

...l/datetime/Plugin/field/widget/DatetimeDatelistWidget.php

core/modules/edit/lib/Drupal/edit/MetadataGenerator.php

...b/Drupal/email/Plugin/field/widget/EmailDefaultWidget.php

core/modules/entity/lib/Drupal/entity/EntityDisplayBase.php

...s/entity/lib/Drupal/entity/EntityDisplayBaseInterface.php

...dules/entity/lib/Drupal/entity/EntityDisplayInterface.php

...odules/entity/lib/Drupal/entity/EntityDisplayModeBase.php

.../lib/Drupal/entity/EntityDisplayModeStorageController.php

...s/entity/lib/Drupal/entity/EntityFormDisplayInterface.php

...ules/entity/lib/Drupal/entity/EntityFormModeInterface.php

...ules/entity/lib/Drupal/entity/EntityViewModeInterface.php

...ty/lib/Drupal/entity/Plugin/Core/Entity/EntityDisplay.php