Move hard coded AJAX progress bar classes to a theme function

Added a theme function wrapping the progress bar element.

There is a PR for the cd_tools module to facilitate the new theme function in claro testing
https://github.com/zolhorvath/cd_tools/pull/1.

Re-rolled adding missing core dir.

I reviewed and tested and LGTM

Tested and works as expected: adds a new wrapper with classes ajax-progress ajax-progress-bar unless stable is loaded, then it keeps working with the previous behavior adding the classes to the already existing div with class progress.

@lauriii are we worrying at all about the BC implications of making this change? What if anything are existing themes going to have to do - especially if they don't extend stable (eg bootstrap).

I think we need a change record here too.

Can we get a release notes snippet added to the issue summary for this one, thanks.

1. The patch in #7 no longer applies.

2. +++ b/core/themes/stable/js/ajax.es6.js
   @@ -0,0 +1,17 @@
   + * Provides backwards compatibility layer for Ajax related markup.
   ...
   +  Drupal.theme.ajaxProgressBar = $element =>
   +    $element.addClass('ajax-progress ajax-progress-bar');
   +})(Drupal);
   
   +++ b/core/themes/stable/stable.info.yml
   @@ -298,3 +298,7 @@ libraries-override:
   +libraries-extend:
   +  core/drupal.ajax:
   +    - stable/drupal.ajax
   

   Ooohhhhhhhhh, this is super clever! I'm impressed 👏🤓

Re-rolled #7.

Eslint reports errors even when it is runned with the "passing" rules.

I fixed them only for the files that are affected by this patch.

1. +++ b/core/misc/ajax.es6.js
   @@ -655,9 +655,7 @@
          window.alert(
   -        `An error occurred while attempting to process ${this.options.url}: ${
   -          e.message
   -        }`,
   +        `An error occurred while attempting to process ${this.options.url}: ${e.message}`,
          );
          // For consistency, return a rejected Deferred (i.e., jqXHR's superclass)
          // so that calling code can take appropriate action.
   @@ -750,9 +748,7 @@
   
   @@ -750,9 +748,7 @@
          // the complete response.
          ajax.ajaxing = false;
          window.alert(
   -        `An error occurred while attempting to process ${ajax.options.url}: ${
   -          e.message
   -        }`,
   +        `An error occurred while attempting to process ${ajax.options.url}: ${e.message}`,
          );
        }
   

   ✔️ These changes are out-of-scope, @huzooka did those to fix eslint errors. That's fine IMHO.

2. +++ b/core/themes/stable/js/ajax.es6.js
   @@ -0,0 +1,17 @@
   + * Provides backwards compatibility layer for Ajax related markup.
   

   🤓 Übernit that can be fixed on commit: Ajax-related

Committed bb3791d and pushed to 8.8.x. Thanks!