Merge LuCI PR foor Static Lease DHCP

@jow sorry to tag you here, but this luci PR has been waiting for two years. I and others have found need of it more than once.

Perhaps @feckert could also review, considering how luci has progressed over the last few years.

Found some more minor issues with it. Might be able to look at it tonight, otherwise in 2.5 weeks earliest

Good review - thanks. Addressed and ready.