luci-mod-status: hide iptables firewall status when nft is present

Do not expose the iptables status page as menu item when nftables is present
on the system. Instead add a warning banner to the nftables status page
directing the user to the hidden iptables status page when we encounter
legacy rules on the system.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>