Changed editor title input to treat '(Untitled)' as blank

refs https://github.com/TryGhost/Team/issues/707

- the switch to a hard `'(Untitled)'` when you start typing is jarring and pulls you away from writing to want to set a title
- by changing the input to treat '(Untitled)' as a blank value the placeholder continues to be shown so flow isn't broken. Post validation and title display elsewhere in the app is unaffected because we're still setting `'(Untitled')` under the hood, it's only the editor display that changes

ghost/admin/app/components/gh-koenig-editor.hbs

@@ -10,7 +10,7 @@
             @placeholder={{@titlePlaceholder}}
             @tabindex="1"
             @autoExpand=".gh-koenig-editor"
-            @value={{readonly @title}}
+            @value={{readonly this.title}}
             @input={{this.updateTitle}}
             @focus-out={{optional @onTitleBlur}}
             @keyDown={{this.onTitleKeydown}}

ghost/admin/app/components/gh-koenig-editor.js

@@ -7,6 +7,10 @@ export default class GhKoenigEditorComponent extends Component {
     koenigEditor = null;
     mousedownY = 0;
 
+    get title() {
+        return this.args.title === '(Untitled)' ? '' : this.args.title;
+    }
+
     @action
     registerElement(element) {
         this.containerElement = element;