Bug 17905: FIX CSRF in member-flags

Bug 17905: FIX CSRF in member-flags

If an attacker can get an authenticated Koha user to visit their page
with the url below, privilege escalation is possible

The exploit can be simulated triggering
    /cgi-bin/koha/members/member-flags.pl?member=42&newflags=1&flag=superlibrarian

Test plan:
Trigger the url above
=> Without this patch, 42 is now superlibrarian
=> With this patch, you will get the "Wrong CSRF token" error.

This vulnerability has been reported by MDSec.

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>