Refactored App into a class component

refs https://github.com/TryGhost/Team/issues/1665

- We need to hook into react element lifecycle events to initialize the search incdex (powered by elasticlunr).

ghost/sodo-search/src/App.js

@@ -3,16 +3,16 @@ import './App.css';
 import AppContext from './AppContext';
 import PopupModal from './components/PopupModal';
 
-function App() {
-    return (
-        <AppContext.Provider value={{
-            page: 'search',
-            showPopup: true,
-            onAction: () => {}
-        }}>
-            <PopupModal />
-        </AppContext.Provider>
-    );
+export default class App extends React.Component {
+    render() {
+        return (
+            <AppContext.Provider value={{
+                page: 'search',
+                showPopup: true,
+                onAction: () => {}
+            }}>
+                <PopupModal />
+            </AppContext.Provider>
+        );
+    }
 }
-
-export default App;