Double underscores in views css classes are now preserved

Views no longer sanitises double underscores in css classes defined on the view display. In Drupal 8.5.x and before, a class like views__display would be replaced with views-display. in Drupal 8.6.0 and onwards, a class like views__display will remain views__display. Drupal 8 implements a backward compatibility layer so the view will still have the views--display class. This will be changed in the #2977950: Move the bc layer for views UI CSS classes from views to stable9.