FIX: A few more syntax fixes for button actions (#162)

assets/javascripts/discourse/templates/admin/plugins-discourse-subscriptions-coupons.hbs

@@ -30,7 +30,7 @@
             </td>
             <td>
               <DButton
-                @action={{this.deleteCoupon}}
+                @action={{action "deleteCoupon"}}
                 @actionParam={{coupon}}
                 @icon="trash-alt"
                 class="btn-danger btn btn-icon btn-no-text"
@@ -44,7 +44,7 @@
 
   {{#unless creating}}
     <DButton
-      @action={{this.openCreateForm}}
+      @action={{action "openCreateForm"}}
       @label="discourse_subscriptions.admin.coupons.create"
       @title="discourse_subscriptions.admin.coupons.create"
       @icon="plus"

assets/javascripts/discourse/templates/admin/plugins-discourse-subscriptions-products-show.hbs

@@ -135,7 +135,7 @@
 {{/unless}}
 
 <div class="pull-right">
-  <DButton @label="cancel" @action={{this.cancelProduct}} @icon="times" />
+  <DButton @label="cancel" @action={{action "cancelProduct"}} @icon="times" />
 
   {{#if model.product.isNew}}
     <DButton