[FIX] website: load wysiwyg in iframe only for publishers

See merge commit for more information.

task-2687506

addons/website/static/src/client_actions/website_preview/website_preview.xml

@@ -23,7 +23,8 @@
                 class="o_iframe"
                 t-ref="iframe"
                 t-on-load="_onPageLoaded"
-                is-ready="false"/>
+                is-ready="false"
+                t-att-data-load-wysiwyg="this.websiteService.isPublisher ? 'true': 'false'"/>
         </div>
         <WebsiteEditorComponent t-if="websiteContext.edition"
             reloadIframe.bind="this.reloadIframe"

addons/website/static/src/js/content/website_root_instance.js

@@ -5,7 +5,9 @@ import { WebsiteRoot } from "./website_root";
 import { loadWysiwyg } from "web_editor.loader";
 
 export default createPublicRoot(WebsiteRoot).then(rootInstance => {
-    if (window.parent !== window) {
+    // This data attribute is set by the WebsitePreview client action for a
+    // publisher user.
+    if (window.frameElement && window.frameElement.dataset.loadWysiwyg) {
         loadWysiwyg(['website.compiled_assets_wysiwyg']).then(() => {
             window.dispatchEvent(new CustomEvent('PUBLIC-ROOT-READY', {detail: {rootInstance}}));
         });