Change criteria for Zurich stats for moderation/resolution

As described in https://github.com/mysociety/FixMyStreet-Commercial/issues/459

Could do with sanity-check from someone who knows FMS, and perhaps some user-acceptance that the new stats criteria are correct for Zurich spec!

Note that I've not implemented the admin_log feature that @dracos mentioned in that ticket, this feels like a useful enhancement as opposed to a showstopper for this issue?