Update 'element-invisible' and 'element-hidden' to valid Drupal 8 CSS classes

Problem/Motivation

We are still using old CSS class names in Drupal 8:

$ git grep -n  "element-invisible"
core/modules/views_ui/js/views-admin.js:562:      this.operator.find('label').add('div.description').addClass('element-invisible');

$ git grep -n  "element-hidden"
core/modules/ckeditor/src/Plugin/Editor/CKEditor.php:218:      '#markup' => '<div id="ckeditor-hidden" class="element-hidden"></div>',
core/modules/quickedit/js/quickedit.js:408:    var $el = $('<div id="' + id + '" class="element-hidden"></div>').appendTo('body');

element-hidden and element-invisible no longer exist in Drupal, they have been simplified, see this change notice.

Proposed resolution

Rename them:
element-hidden -> hidden
element-invisible -> visually-hidden

Remaining tasks

patch

User interface changes

none

API changes

none