Bug 35752: Pass CSRF token to del_form

Bug 35752: Pass CSRF token to del_form

To test:

1. Have multiple additional content items ( news/HTML customization/Pages )
2. On one of the pages ( news/HTML customization/Pages ) select more than one item and try to delete using the 'Delete selected' button
3. You are redirected but nothing happens.
4. APPLY PATCH
5. Try again, this time you should be able to successfully delete additional-contents.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>