Use Zeitwerk instead of the classic autoloader

In Rails 6.1, the classic autoloader is deprecated.

We were getting an error because we were using `autoload` in the
ActiveStorage plugin for CKEditor:

expected file app/lib/ckeditor/backend/active_storage.rb to define
constant Ckeditor::Backend::ActiveStorage

So we're removing the line causing the error.

Finally, we can now restore all the tests that that failed sometimes
with the classic autoloader and that we modified in commits 2af1fc72f
and 8ba37b295.

config/environments/development.rb

@@ -72,6 +72,7 @@ Rails.application.configure do
   # Annotate rendered view with file names.
   # config.action_view.annotate_rendered_view_with_filenames = true
 
+  config.eager_load_paths << "#{Rails.root}/spec/mailers/previews"
   config.action_mailer.preview_path = "#{Rails.root}/spec/mailers/previews"
 
   # Limit size of local logs