Fixed preview background flash when switching from none -> all/invite

no issue

- flash was occurring because as soon as the setting was changed we switched to showing the portal preview but at that time the preview URL is still set to an old url value. The url doesn't get changed until the settings save completes which gives enough time for a non-portal page to load before being replaced
- added a `switchFromNoneTask` that is triggered when switching away from a saved none value and updated the template to stay on the "disabled" view until the save has completed so we don't trigger multiple page loads

ghost/admin/app/controllers/settings/membership.js

@@ -111,7 +111,7 @@ export default class MembersAccessController extends Controller {
             // when saved value is 'none' the server won't inject the portal script
             // to work around that and show the expected portal preview we save and
             // force a refresh
-            await this.saveSettingsTask.perform({forceRefresh: true});
+            await this.switchFromNoneTask.perform();
         } else {
             this.updatePortalPreview();
         }
@@ -257,6 +257,11 @@ export default class MembersAccessController extends Controller {
         this.resizePortalPreviewTask.cancelAll();
     }
 
+    @task
+    *switchFromNoneTask() {
+        return yield this.saveSettingsTask.perform({forceRefresh: true});
+    }
+
     @task({restartable: true})
     *resizePortalPreviewTask() {
         if (this.portalPreviewIframe && this.portalPreviewIframe.contentWindow) {

ghost/admin/app/templates/settings/membership.hbs

@@ -48,7 +48,7 @@
             </div>
             <div class="gh-setting-members-portalpreview">
                 <div class="gh-setting-members-portal-mock">
-                    {{#if (eq this.settings.membersSignupAccess 'none')}}
+                    {{#if (or (eq this.settings.membersSignupAccess 'none') this.switchFromNoneTask.isRunning)}}
                         <div class="gh-setting-members-portal-disabled">
                             <span class="lightgrey">{{svg-jar "portal-logo-stroke"}}</span>
                             <h4>Portal disabled</h4>