Restore the filebrowser icon for file fields.

Files taken from https://github.com/wardi/django-filebrowser-no-grappelli,
which does work properly but isn't in sync anymore with the sehmaschine development line.
Make sure popup windows hide the header/footer.

The filebrowser did pass the 'pop' parameter,
but didn't pass that to the Django-admin templates.
Fix collapse function at upload form.

The `collapse close` value was present in the HTML, but the Django admin
jQuery scripts were not loaded properly, hence the help box was always
visible.