Add translations for messages and remove old placeholder translation

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

@@ -6,5 +6,5 @@ angular.module("admin.payments").controller "PaymentCtrl", ($scope, Payment, Sta
   $scope.submitPayment = () ->
     return false if $scope.submitted == true
     $scope.submitted = true
-    StatusMessage.display 'progress', t("submitting_payment")
+    StatusMessage.display 'progress', t("spree.admin.payments.source_forms.stripe.submitting_payment")
     Payment.purchase()

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

@@ -43,5 +43,5 @@ angular.module('admin.payments').factory 'Payment', (AdminStripeElements, curren
       PaymentResource.create({order_id: munged.order_id}, munged, (response, headers, status)=>
         $window.location.pathname = "/admin/orders/" + munged.order_id + "/payments"
       , (response) ->
-        StatusMessage.display 'error', t("error saving payment")
+        StatusMessage.display 'error', t("spree.admin.payments.source_forms.stripe.error_saving_payment")
       )

config/locales/en.yml

@@ -2612,7 +2612,8 @@ See the %{link} to find out more about %{sitename}'s features and to start using
       payments:
         source_forms:
           stripe:
-            no_payment_via_admin_backend: Creating Stripe-based payments from the admin backend is not possible at this time
+            error_saving_payment: Error saving payment
+            submitting_payment: Submitting payment...
       products:
         new:
           title: 'New Product'

config/locales/en_GB.yml

@@ -2380,7 +2380,9 @@ en_GB:
       payments:
         source_forms:
           stripe:
-            no_payment_via_admin_backend: Creating Stripe-based payments from the admin backend is not possible at this time
+            error_saving_payment: Error saving payment
+            submitting_payment: Submitting payment...
+
       products:
         new:
           title: 'New Product'