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

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

app/views/settings/_settings_nav.html.erb

@@ -8,7 +8,7 @@ nav_sections = [
       { label: t(".preferences_label"), path: settings_preferences_path, icon: "bolt" },
       { label: t(".profile_label"), path: settings_profile_path, icon: "circle-user" },
       { label: t(".security_label"), path: settings_security_path, icon: "shield-check" },
-      { label: t(".payment_label"), path: settings_payment_path, icon: "circle-dollar-sign", if: !self_hosted? }
+      { label: t(".payment_label"), path: settings_payment_path, icon: "circle-dollar-sign", if: !self_hosted? && Current.family.can_manage_subscription? }
     ]
   },
   {