luci-app-firewall: use new ipmask validation types when applicable

This allows for address specifications like "fdca:1234:0123::abcd/::ffff:ffff:ffff:ffff"
which only match the last 64 bits of an address. This syntax is legal and already supported
by iptables and firewall3.

Fixes https://bugs.lede-project.org/index.php?do=details&task_id=417

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