Convert custom_block.js into a library

Oh, we should actually wait until the parent issue has landed — I should have created this issue as postponed, sorry! :-/

Once that is in, your patch will probably work - we will just need to change the #attached syntax.

#2203407: Replace #attached library array values with provider-namespaced strings has landed, so this needs a re-roll now.

1. +++ b/core/modules/block/custom_block/custom_block.libraries.yml
   @@ -0,0 +1,5 @@
   +  version: 1.0
   

   For all libraries of modules in core, the value of version should be VERSION

2. +++ b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
   @@ -140,9 +140,6 @@ public function form(array $form, array &$form_state) {
   -      '#attached' => array(
   ...
          '#access' => $block->isNewRevision() || $account->hasPermission('administer blocks'),
   

   It looks like the JS was only conditionally loaded previously?

   #access causes the entire element to not be rendered, so #attached libraries are not loaded if #access is FALSE.

3. We're missing the declaration of dependencies for the library. AFAICS, the dependencies are:

   - core/drupal.form
   - core/drupal
   - core/jquery

Thanks!