luci-app-firewall: redirect to overview page on zone details save

This fixes an inconsistency because on the interface configuration if
you press Save&Apply it will go back to overview page.  It is also the
case with "Firewall - Traffic Rules" details.  On firewall zone it only
goes back to firewall zone-detail. Same behaviour on all pages is a good
user experience.

Signed-off-by: Florian Eckert <Eckert.Florian@googlemail.com>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>