Move app/server to flipper-server-core

Summary: moved `app/src/server` to `flipper-server-core/src` and fixed any fallout from that (aka integration points I missed on the preparing diffs).

Reviewed By: passy

Differential Revision: D31541378

fbshipit-source-id: 8a7e0169ebefa515781f6e5e0f7b926415d4b7e9

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

@@ -7,6 +7,10 @@
  * @format
  */
 
-export function helloWorld() {
-  return true;
-}
+export {
+  FlipperServerConfig,
+  getFlipperServerConfig,
+  setFlipperServerConfig,
+} from './FlipperServerConfig';
+
+export {FlipperServerImpl} from './FlipperServerImpl';