Bug 34368: Add CSRF token to Content Management pages

Bug 34368: Add CSRF token to Content Management pages

This change adds a CSRF token to the Content Management pages
at additional-contents.pl.

Test plan:
0. Apply patch
1. koha-plack --restart kohadev
2. Try to add "News", "HTML customizations", and "Pages".
3. Try to delete these new content entries
4. Note that you were successful in your endeavours

JD amended patch: remove empty line removal (no need to create
unecessary conflicts)

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>