Move keychain storage to server, some constants cleanup

Summary: This diff moves keychain storage to the server. Figured to leave request logic itself in the UI-core, as basically all use cases happen there, except for streaming download for mobile build plugin, so sending the requests from the backend doesn't really seem to add value, unless we run into some CORS issues later.

Reviewed By: passy

Differential Revision: D32596715

fbshipit-source-id: f5ab9d794f91a6eb8a8dc07ae723bf2890726771

desktop/flipper-ui-core/src/fb-stubs/constants.tsx

@@ -42,4 +42,7 @@ export default Object.freeze({
   },
 
   SUPPORT_GROUPS: [],
+
+  INTERN_URL: '',
+  INTERNGRAPH_API: '',
 });