Fix Loading page

Summary:
The existing loading page was not behaving the way it was intended. The previous implementation triggered a page reload which made the whole retry mechanism useless.

Instead, a new endpoint was defined to expose whether the server is ready or not. Use this instead as a way of knowing whether we are good to reload the page.

Reviewed By: passy

Differential Revision: D49314749

fbshipit-source-id: eb67765d7deab8610fa5d31e710070da43a18c1c

desktop/flipper-server-core/src/server/startServer.tsx

@@ -128,6 +128,11 @@ async function startHTTPServer(config: Config): Promise<{
     });
   });
 
+  app.get('/ready', (_req, res) => {
+    res.setHeader('Content-Type', 'application/json');
+    res.end(JSON.stringify({isReady}));
+  });
+
   app.get('/health', (_req, res) => {
     res.end('flipper-ok');
   });