Issue #2046303 by kim.pepper: Convert aggregator_form_category() to FormInterface.

core/modules/aggregator/aggregator.admin.inc

core/modules/aggregator/aggregator.module

core/modules/aggregator/aggregator.routing.yml

core/modules/aggregator/aggregator.services.yml

...gator/lib/Drupal/aggregator/CategoryStorageController.php

.../Drupal/aggregator/CategoryStorageControllerInterface.php

...lib/Drupal/aggregator/Controller/AggregatorController.php

...gregator/lib/Drupal/aggregator/Form/CategoryAdminForm.php

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

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

core/modules/block/lib/Drupal/block/Tests/BlockTest.php