Accessibility checker for ckeditor

The ckeditor accessibility checker works nicely. We should add it to the filtered text format

• https://www.drupal.org/project/ckeditor_a11ychecker

• https://www.drupal.org/project/ckeditor_balloonpanel

• https://ckeditor.com/cke4/addon/balloonpanel

• https://ckeditor.com/cke4/addon/a11ychecker

• add modules / libraries to the repo

• do the config