Added help/about box to wysiwyg editor

- To display license info along with shortcuts.
- Extracted out plain layout from 503 error page.
- Added tests to ensure license references are as expected.

resources/js/wysiwyg/plugins-about.js

@@ -0,0 +1,29 @@
+/**
+ * @param {Editor} editor
+ * @param {String} url
+ */
+function register(editor, url) {
+
+    const aboutDialog = {
+        title: 'About the WYSIWYG Editor',
+        url: window.baseUrl('/help/wysiwyg'),
+    };
+
+    editor.ui.registry.addButton('about', {
+        icon: 'help',
+        tooltip: 'About the editor',
+        onAction() {
+            tinymce.activeEditor.windowManager.openUrl(aboutDialog);
+        }
+    });
+
+}
+
+
+/**
+ * @param {WysiwygConfigOptions} options
+ * @return {register}
+ */
+export function getPlugin(options) {
+    return register;
+}