Added basic reset to design preview on nav bar interaction

refs https://github.com/TryGhost/Team/issues/1130

- when opening/closing sections or interacting with the general design or custom theme settings we want to switch back to showing the preview so the sidebar acts more like typical navigation and so form changes show their effect

ghost/admin/app/components/gh-html-iframe.hbs

@@ -1,5 +1,5 @@
 <iframe
-    {{did-insert this.registerIframe}}
+    {{did-insert this.replaceIframeContents}}
     {{did-update this.replaceIframeContents @html}}
     ...attributes>
 </iframe>

ghost/admin/app/components/gh-html-iframe.js

@@ -2,19 +2,12 @@ import Component from '@glimmer/component';
 import {action} from '@ember/object';
 
 export default class GhHtmlIframeComponent extends Component {
-    iframe = null;
-
     @action
-    registerIframe(iframe) {
-        this.iframe = iframe;
-    }
-
-    @action
-    replaceIframeContents(iframe, html) {
-        if (this.iframe) {
-            this.iframe.contentWindow.document.open();
-            this.iframe.contentWindow.document.write(html);
-            this.iframe.contentWindow.document.close();
+    replaceIframeContents(iframe) {
+        if (iframe) {
+            iframe.contentWindow.document.open();
+            iframe.contentWindow.document.write(this.args.html);
+            iframe.contentWindow.document.close();
         }
     }
 }

ghost/admin/app/components/gh-nav-menu/design.hbs

@@ -2,7 +2,7 @@
     <LinkTo @route="settings" class="gh-nav-menu-title">{{svg-jar "arrow-left-tail"}} Site design</LinkTo>
 </header>
 <section class="gh-nav-body">
-    <div class="gh-nav-top">
+    <div class="gh-nav-top" {{on "click" this.transitionBackToIndex}}>
 
         <div class="pl6 pt4 pr6">
             <button type="button" {{on "click" (fn this.toggleSection "brand")}} class="flex flex-row w-100">

ghost/admin/app/components/gh-nav-menu/design.js

@@ -6,10 +6,11 @@ import {task} from 'ember-concurrency-decorators';
 import {tracked} from '@glimmer/tracking';
 
 export default class DesignMenuComponent extends Component {
+    @service config;
     @service customThemeSettings;
+    @service router;
     @service settings;
     @service themeManagement;
-    @service config;
 
     @tracked openSections = new TrackedSet();
 
@@ -44,4 +45,11 @@ export default class DesignMenuComponent extends Component {
     *fetchThemeSettingsTask() {
         yield this.customThemeSettings.load();
     }
+
+    @action
+    transitionBackToIndex() {
+        if (this.router.currentRouteName !== 'settings.design.index') {
+            this.router.transitionTo('settings.design.index');
+        }
+    }
 }