(core) Migrate to SRP and add change password dialog

Summary:
Moves some auth-related UI components, like MFAConfig, out
of core, and adds a new ChangePasswordDialog component for
allowing direct password changes, replacing the old reset password
link to hosted Cognito.

Updates all MFA endpoints to use SRP for authentication.

Also refactors MFAConfig into smaller files, and polishes up some parts
of the UI to be more consistent with the login pages.

Test Plan: New server and deployment tests. Updated existing tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3311

app/client/models/gristUrlState.ts

@@ -84,13 +84,6 @@ export function getLoginOrSignupUrl(nextUrl: string = _getCurrentUrl()): string
   return _getLoginLogoutUrl('signin', nextUrl);
 }
 
-// Get url for the reset password page.
-export function getResetPwdUrl(): string {
-  const startUrl = new URL(window.location.href);
-  startUrl.pathname = '/resetPassword';
-  return startUrl.href;
-}
-
 // Returns the URL for the "you are signed out" page.
 export function getSignedOutUrl(): string { return getMainOrgUrl() + "signed-out"; }