Add a directive attribute 'selected' that update AdminStripeElements when true

Filled with `PaymentCtrl.isSelected(payment_method_id)`

Therefore, each time a stripe payment is selected, the one that is actually selected is linked to `AdminStripeElements.card` and `AdminStripeElements.stripe`

app/assets/javascripts/admin/payments/controllers/payment.js.coffee

@@ -8,3 +8,6 @@ angular.module("admin.payments").controller "PaymentCtrl", ($scope, Payment, Sta
     $scope.submitted = true
     StatusMessage.display 'progress', t("spree.admin.payments.source_forms.stripe.submitting_payment")
     Payment.purchase()
+
+  $scope.isSelected = (payment_method_id) ->
+    return parseInt($scope.form_data.payment_method) == payment_method_id

app/assets/javascripts/admin/payments/directives/stripe_elements.js.coffee

@@ -4,7 +4,9 @@ angular.module('admin.payments').directive "stripeElements", ($injector, AdminSt
              <div id='card-element' class='card-element'></div>\
              <div class='error card-errors'></div>\
              </label>"
-
+  scope:
+    selected: "="
+  
   link: (scope, elem, attr)->
     if $injector.has('stripeObject')
       stripe = $injector.get('stripeObject')
@@ -32,5 +34,7 @@ angular.module('admin.payments').directive "stripeElements", ($injector, AdminSt
 
         return
 
-      AdminStripeElements.stripe = stripe
-      AdminStripeElements.card = card
+      scope.$watch "selected", (value) ->
+        if (value)
+          AdminStripeElements.stripe = stripe
+          AdminStripeElements.card = card 

app/views/spree/admin/payments/source_forms/_stripe.html.haml

@@ -16,4 +16,4 @@
     .three.columns
       = label_tag :card_details, t(:card_details)
     .nine.columns
-      %stripe-elements
+      %stripe-elements{selected: "isSelected(#{payment_method.id})"}

app/views/spree/admin/payments/source_forms/_stripe_sca.html.haml

@@ -15,4 +15,4 @@
     .three.columns
       = label_tag :card_details, t(:card_details)
     .nine.columns
-      %stripe-elements
+      %stripe-elements{selected: "isSelected(#{payment_method.id})"}