SSO: allow sign-in with an alternate identifier when it differs from NameID

Since version 2.48.0, the email typed on the Hudu login page has to match the username the identity provider sends back. For organizations where users have a separate sign-in username and primary email address, users who enter their primary email now get “SSO authentication failed”, even though they signed in to their identity provider successfully. Before 2.48, signing in with the primary email worked.

Request: add an option for users to sign in with their primary email address when it differs from their SSO username, without admins having to change their identity provider setup or update every Hudu user account.