luci-mod-network: validate DHCP leasetime input

luci-mod-network: validate DHCP leasetime input

Without validation, `dnsmasq` can fail silently if users enter
invalid leasetime input. This change adds input validation to
align user input with the backend parsing logic. Whilst it does
not enforce the >120 seconds requirement, this is documented
in the field description and handled by `dnsmasq`'s `option.c`,
which adjusts values <120 to meet this minimum.

Signed-off-by: Lyall Beveridge <lyall.beveridge@morsemicro.com>