Issue #2059245 by tim.plunkett, msonnabaum: Add a FormBase class containing useful methods.

core/core.services.yml

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

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

.../lib/Drupal/Core/Entity/EntityFormControllerInterface.php

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

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

core/lib/Drupal/Core/Form/ConfirmFormBase.php

core/lib/Drupal/Core/Form/FormBase.php

...ules/action/lib/Drupal/action/ActionAddFormController.php

...les/action/lib/Drupal/action/ActionFormControllerBase.php

...regator/lib/Drupal/aggregator/Form/CategoryDeleteForm.php

...es/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php

core/modules/ban/lib/Drupal/ban/Form/BanDelete.php

...ib/Drupal/custom_block/Form/CustomBlockTypeDeleteForm.php

core/modules/block/lib/Drupal/block/BlockFormController.php

.../tests/lib/Drupal/block/Tests/BlockFormControllerTest.php

core/modules/config/lib/Drupal/config/Form/ConfigSync.php

...ntity/lib/Drupal/entity/Form/EntityDisplayModeAddForm.php

...tity/lib/Drupal/entity/Form/EntityDisplayModeFormBase.php

...les/field_ui/lib/Drupal/field_ui/Form/FieldDeleteForm.php