Issue #2062439 by tim.plunkett, larowlan: Provide listing of custom block entities.

...modules/block/custom_block/custom_block.local_actions.yml

core/modules/block/custom_block/custom_block.module

core/modules/block/custom_block/custom_block.pages.inc

core/modules/block/custom_block/custom_block.routing.yml

...ock/lib/Drupal/custom_block/CustomBlockFormController.php

...ock/lib/Drupal/custom_block/CustomBlockListController.php

...lib/Drupal/custom_block/CustomBlockTypeFormController.php

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

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

...ib/Drupal/custom_block/Plugin/Core/Entity/CustomBlock.php

...lib/Drupal/custom_block/Tests/CustomBlockCreationTest.php

...ck/lib/Drupal/custom_block/Tests/CustomBlockFieldTest.php

...ock/lib/Drupal/custom_block/Tests/CustomBlockListTest.php

...ock/lib/Drupal/custom_block/Tests/CustomBlockTypeTest.php