Add CKEditor support for ActiveStorage

ActiveStorage support was added to CKEditor in version 5.1.0. However,
we can't upgrade to version 5.0.0 or later because it only supports
loading CKEditor via CDN.

So we're copying the code related to ActiveStorage instead. I tried to
move it to the `vendor/` folder, but couldn't find a way to load it from
there and if I found one I wouldn't be sure it'd work on production
environments.

.rubocop.yml

@@ -8,6 +8,7 @@ AllCops:
   DisplayStyleGuide: true
   Exclude:
     - "db/schema.rb"
+    - "lib/ckeditor/backend/active_storage.rb"
   DisabledByDefault: true
 
 Bundler/DuplicatedGem: