Added ctrl/cmd+s save on design settings screen

refs https://github.com/TryGhost/Team/issues/1149

- use ember-keyboard's `{{on-key}}` modifier to attach keyboard event listener when the design settings index screen is shown - ensures that ctrl/cmd+s on the change-theme screen won't trigger an unexpected save
- blur active element when saving so the preview is updated and reflects what has just been saved (text fields only update preview on blur)

ghost/admin/app/controllers/settings/design/index.js

@@ -1,5 +1,6 @@
 import Controller from '@ember/controller';
 import {action} from '@ember/object';
+import {next} from '@ember/runloop';
 import {inject as service} from '@ember/service';
 import {task} from 'ember-concurrency-decorators';
 import {tracked} from '@glimmer/tracking';
@@ -26,6 +27,12 @@ export default class SettingsDesignIndexController extends Controller {
         this.previewSize = size;
     }
 
+    @action
+    saveFromKeyboard() {
+        document.activeElement.blur?.();
+        return this.saveTask.perform();
+    }
+
     @task
     *saveTask() {
         try {

ghost/admin/app/templates/settings/design/index.hbs

@@ -1,4 +1,4 @@
-<section class="gh-canvas gh-design">
+<section class="gh-canvas gh-design" {{on-key "cmd+s" this.saveFromKeyboard}}>
     <GhCanvasHeader class="gh-canvas-header">
         <h2 class="gh-canvas-title">Site design</h2>
         <section class="view-actions">