luci-mod-admin-full: host-specific dhcp lease time option for static hosts

Enable setting a host-specific lease time for static hosts.
Format is similar as for the default lease time: e.g. 2m, 12h, 3d, infinite

Default lease time is used for all hosts without host-specific definition.

Support for the option was added to Openwrt trunk by r48801:
https://dev.openwrt.org/changeset/48801

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>