js: fixup gettext in django 3.1

For some reason in Django 3.1 django.gettext is not available anymore in the scope,
it's available as window.gettext though. Update translate.js to work
with both locations.

This was already reported here:
geex-arts#149

Fix tikservices#25

jet/static/jet/js/src/utils/translate.js

@@ -1,6 +1,7 @@
 module.exports = function(str) {
-    if (window.django == undefined) {
+    const gettext = django.gettext || window.gettext;
+    if (gettext == undefined) {
         return str;
     }
-    return django.gettext(str);
+    return gettext(str);
 };