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/GK.tsx

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+export const GK = {
+  get(_name: string): boolean {
+    return false;
+  },
+};