Left rail log in icon decided by atom, cache user profile

Summary:
Previously the left rail state was decided by prescense of user profile but we are moving to logged in state being based on the atom.

As a result we need to cache the user profile so we have something to display when user not on vpn on startup

Reviewed By: aigoncharov

Differential Revision: D44502477

fbshipit-source-id: 11462d24c773d6d364e844b4f606e124e5278348

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

@@ -10,10 +10,14 @@
 import {GraphFileUpload, User} from 'flipper-common';
 import {Atom, createState} from 'flipper-plugin';
 
-export async function getUser(): Promise<User | null> {
+export async function fetchUser(): Promise<User | null> {
   throw new Error('Feature not implemented');
 }
 
+export function getCachedUser(): User | null {
+  return null;
+}
+
 export async function internGraphPOSTAPIRequest(
   _endpoint: string,
   _formFields: {