Fix authentication on "untenanted" controllers

trying out the name "disallow_account_scope" for this, but I don't
think it's quite right yet.

app/controllers/sessions_controller.rb

@@ -4,7 +4,7 @@ class SessionsController < ApplicationController
   SIGNUP_PASSWORD = Rails.env.local? ? "testpassword" : Rails.application.credentials.account_signup_http_basic_auth.password
   http_basic_authenticate_with name: SIGNUP_USERNAME, password: SIGNUP_PASSWORD, realm: "Fizzy Signup", only: :create, unless: -> { Identity.exists?(email_address: email_address) }
 
-  require_untenanted_access
+  disallow_account_scope
   require_unauthenticated_access except: :destroy
   rate_limit to: 10, within: 3.minutes, only: :create, with: -> { redirect_to new_session_path, alert: "Try again later." }