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-ui-browser/src/index.tsx

@@ -41,7 +41,10 @@ async function start() {
     token = manifest.token;
   }
 
-  console.info('Token is available: ' + token !== undefined && token?.length);
+  console.info(
+    '[flipper-client][ui-browser] Token is available: ',
+    token?.length,
+  );
 
   const openPlugin = params.get('open-plugin');
   if (openPlugin) {