(core) Handle old Grist sessions in MFAConfig

Summary:
Grist sessions created pre-MFA do not store access or
refresh tokens, which means that MFA status from Cognito
can't be loaded without requiring re-authentication. MFAConfig
handles this by requiring security verification as usual, and
checking if it needs to reload MFA status on success. If it does, it'll
close the 2FA setup dialog and reload, which should show the correct
2FA configuration status and buttons.

Test Plan: Updated existing tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3231

app/common/UserAPI.ts

@@ -298,12 +298,13 @@ export type PassVerificationResult = ChallengeRequired | ChallengeNotRequired;
 /**
  * Information about the follow-up authentication challenge.
  */
-interface ChallengeRequired {
+export interface ChallengeRequired {
   isChallengeRequired: true;
+  isAlternateChallengeAvailable: boolean;
   // Session identifier that must be re-used in response to auth challenge.
   session: string;
   challengeName: 'SMS_MFA' | 'SOFTWARE_TOKEN_MFA';
-  // If challenge is 'SMS_MFA', the destination number that the verification code was sent.
+  // If SMS MFA is enabled, the destination phone number that codes are sent to.
   deliveryDestination?: string;
 }