Feat: replace billing address component for JH

Allow JH to override the billing address component

ee/app/assets/javascripts/subscriptions/new/components/checkout.vue

@@ -3,7 +3,7 @@ import { mapState } from 'vuex';
 import ProgressBar from 'ee/registrations/components/progress_bar.vue';
 import { STEPS, SUBSCRIPTON_FLOW_STEPS } from 'ee/registrations/constants';
 import { s__ } from '~/locale';
-import BillingAddress from './checkout/billing_address.vue';
+import BillingAddress from 'jh_else_ee/subscriptions/new/components/checkout/billing_address.vue';
 import ConfirmOrder from './checkout/confirm_order.vue';
 import PaymentMethod from './checkout/payment_method.vue';
 import SubscriptionDetails from './checkout/subscription_details.vue';