Fix hidden modal with JavaScript disabled

'appear' animation is preserved with client-side routing.
However, if JavaScript is enabled and the page is rendered on the server-side,
the 'appear' phase may not work as expected because the modal is already
rendered and displayed.
The flick is smoother on more performant hardware and network

src/components/sidebar-modal.js

@@ -99,8 +99,8 @@ class SidebarModalOverlay extends React.Component {
   constructor(props, ...args) {
     super(props, ...args);
     this.state = {
-      isAppearing: false,
-      isVisible: false
+      isAppearing: typeof window === 'undefined',
+      isVisible: typeof window === 'undefined'
     };
   }