add static files

static/admin/js/cancel.js

@@ -0,0 +1,13 @@
+(function($) {
+    'use strict';
+    $(function() {
+        $('.cancel-link').on('click', function(e) {
+            e.preventDefault();
+            if (window.location.search.indexOf('&_popup=1') === -1) {
+                window.history.back(); // Go back if not a popup.
+            } else {
+                window.close(); // Otherwise, close the popup.
+            }
+        });
+    });
+})(django.jQuery);