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

@@ -3,7 +3,8 @@ import './App.css';
 import AppContext from './AppContext';
 import PopupModal from './components/PopupModal';
 
-function App() {
+export default class App extends React.Component {
+    render() {
         return (
             <AppContext.Provider value={{
                 page: 'search',
@@ -14,5 +15,4 @@ function App() {
             </AppContext.Provider>
         );
     }
-
-export default App;
+}