Bring simple signup flow from the fizzy-saas gem

We skip the QB code and we fill external account ids automatically on creation with a sequence

See:
https://github.com/basecamp/fizzy-saas/pull/7

app/controllers/sessions_controller.rb

@@ -22,10 +22,8 @@ class SessionsController < ApplicationController
       magic_link = identity.send_magic_link
       flash[:magic_link_code] = magic_link&.code if Rails.env.development?
       redirect_to session_magic_link_path
-    elsif signups_allowed?
-      Signup.new(email_address: email_address).create_identity
-      session[:return_to_after_authenticating] = saas.new_signup_completion_path
-      redirect_to session_magic_link_path
+    elsif
+      process_new_signup
     end
   end
 
@@ -38,4 +36,10 @@ class SessionsController < ApplicationController
     def email_address
       params.expect(:email_address)
     end
+
+    def process_new_signup
+      Signup.new(email_address: email_address).create_identity
+      session[:return_to_after_authenticating] = new_signup_completion_path
+      redirect_to session_magic_link_path
+    end
 end