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-common/src/index.tsx

@@ -41,3 +41,5 @@ export {
   getErrorFromErrorLike,
 } from './utils/errors';
 export * from './user-session';
+export * from './GK';
+export * from './clientUtils';