Search articles in group_orders form

See foodcoops#143 for previous discussion.

This adds the possibility to filter articles by name when ordering. The plugin list.js was used which had to be modified to not break the form submission.

Further work has to be done when the filtering shall be combined with sorting functionality because the list.js has its own sort logic. Ordering of DOM objects outside of list.js will be overwritten by list.js. That should be addressed in a separate issue if needed.