Force logged in

Summary:
This diff introduces a few changes:
The login sheet is no longer closable via the x or clicking outside the modal, the cancel button is removed
On startup we check if we have a toke and throw up the sheet
if logout occurs for any reason we throw up the sheet

renamed write_keychain to login, to be consstent with the logout method. It does more than write to key chain since it manages the logged in atom also

Reviewed By: aigoncharov

Differential Revision: D44502483

fbshipit-source-id: 1d91d4eaae65ca523a08e205d1ad730d4d4d090f

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

@@ -96,7 +96,9 @@ export async function getPreferredEditorUriScheme(): Promise<string> {
   return 'vscode';
 }
 
-export async function initialize(): Promise<void> {}
+export async function initialize(): Promise<boolean> {
+  return true;
+}
 
 export async function appendAccessTokenToUrl(_url: URL): Promise<string> {
   throw new Error('Implement appendAccessTokenToUrl');