Hide contribution payments from demo user(s) (#738)

* Hide payment contribution options from demo and manually created users

Demo data users and manually created users don't have stripe_customer_id
set on their family, so they should not see payment/contribution options.

Changes:
- Add can_manage_subscription? method to Family::Subscribeable that checks
  for presence of stripe_customer_id
- Guard Settings::PaymentsController to return 403 for users without
  stripe_customer_id
- Guard SubscriptionsController#show action (Stripe portal redirect) for
  users without stripe_customer_id
- Update settings navigation to hide the payment link when
  stripe_customer_id is not present
- Add tests for the new behavior

* Fix broken test

---------

Co-authored-by: Claude <noreply@anthropic.com>

app/controllers/settings/payments_controller.rb

@@ -1,6 +1,8 @@
 class Settings::PaymentsController < ApplicationController
   layout "settings"
 
+  guard_feature unless: -> { Current.family.can_manage_subscription? }
+
   def show
     @family = Current.family
   end