Bug 29931: Check cookie status before continuing

Bug 29931: Check cookie status before continuing

Test plan:
Logout from staff.
Try to run plugins-enable (you should have some active plugin).
Like: https://yourserver:staffport/cgi-bin/koha/plugins/plugins-enable.pl?class=Koha::Plugin::Test&method=enable
Replace class and method as appropriate.
Verify that with this patch, you will be redirected to 401 page.

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