luci-base: show default forwarding policy in zone forwarding list

The zone forwarding list widget misleadingly displays the intra-zone
forwarding policy as default policy action when no forwardings exist
instead of the appropriate global defaults forwarding policy which is
the one applied to inter-zone forwarded traffic.

Fix the issue by displaying the defaults policy and not the per-zone
policy to match what the firewall implementation is actually doing.

Fixes: #2213
Signed-off-by: Jo-Philipp Wich <jo@mein.io>