Bug 34369: Require CSRF token for updating system preferences

Bug 34369: Require CSRF token for updating system preferences

This patch adds the requirements that updating a system preference
requires a CSRF token. (Also, adding and deleting local system preferences.)

0. Apply patch
1. koha-plack --reload kohadev
2. Add local system preference
3. Update local system preference
4. Delete local system preference
5. Update normal system preference
6. Note no errors

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