Add trailing slash to model URLs in the correct way (#224)

frontend/vre/utils/api.model.js

@@ -1,14 +1,13 @@
 import Backbone from 'backbone';
+import { modelSlashUrl } from '@uu-cdh/backbone-util';
 import { objectAsUrlParams } from './generic-functions';
 
 /**
  * Generic subclass that appends a slash to the model URL.
  * This is required for interop with Django REST Framework.
  */
 export var APIModel = Backbone.Model.extend({
-    url: function() {
-        return Backbone.Model.prototype.url.call(this) + '/';
-    },
+    url: modelSlashUrl(),
 });
 
 /**
@@ -27,4 +26,4 @@ export var APICollection = Backbone.Collection.extend({
         }).value();
         return this.fetch(fetchOptions);
     },
-});
+});