luci-app-statistics: add dhcpleases

Add the dhcpleases plugin:
https://github.com/openwrt/packages/pull/14204

Added-Frontend-by: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: Nick Hainke <vincent@systemli.org>

[Minor changes to graph: alt_autoscale, precision, label]
Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>