Issue #2078341 by tim.plunkett: Restructure Block UI and custom block pages.

core/modules/block/block.module

core/modules/block/custom_block/custom_block.info.yml

core/modules/block/custom_block/custom_block.module

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

...stom_block/lib/Drupal/custom_block/Entity/CustomBlock.php

..._block/lib/Drupal/custom_block/Entity/CustomBlockType.php

...ck/lib/Drupal/custom_block/Form/CustomBlockDeleteForm.php

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

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

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

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