Add feature_helper to tidy up feature flag queries

app/assets/javascripts/helpers/feature_helper.js

+import Cookies from 'js-cookie';
+
+function isNewRepo() {
+  return Cookies.get('new_nav') === 'true';
+}
+
+const FeatureHelper = {
+  isNewRepo,
+};
+
+export default FeatureHelper;