Add user ratings

Summary:
Adds a star rating to flipper internal.

Based on the nuclide behaviour:
We get an event emitted when a user changes their rating.
And we also get the rating with every ping event, which is fired every minute, if flipper is the primary window.

The only thing I don't like that much is that this doesn't actually say anywhere what the stars are for.

Reviewed By: passy

Differential Revision: D16420281

fbshipit-source-id: 69a52f64058955d7cd068215478e95c554cb9ed4

src/fb-stubs/config.js

@@ -9,4 +9,5 @@ export default {
   updateServer: 'https://www.facebook.com/fbflipper/public/latest.json',
   bugReportButtonVisible: false,
   showLogin: false,
+  showFlipperRating: false,
 };