Remove empty div tag containing the navbar.

In preliminary, the appearance and behavior of the navbar was not affected when the `<nav>` element was not wrapped, and an empty `<div>` element doesn't do anything anyway. I'm also pretty sure that the `<nav>` tag in HTML5 is basically a div with a fancy name, so it can do everything a div can.

ui/src/components/navbar.tsx

@@ -84,7 +84,7 @@ export class Navbar extends Component<any, NavbarState> {
   }
 
   render() {
-    return <div>{this.navbar()}</div>;
+    return this.navbar();
   }
 
   componentWillUnmount() {