luci-mod-admin-full: Add mesh_fwding support

* make it only visible for 802.11s mesh-networks
* have it enabled by default

Signed-off-by: Sven Roederer <freifunk@it-solutions.geroedel.de>
[add empty line before meshfwd assignment]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>