Display only latest editions of CIPs in admin

www.agileplannerapp.com/boards/173808/cards/5997
fixes: https://govuk.zendesk.com/tickets/823384

editors can get confused looking at published as well as in-progress editions in the CIP tab of an organisation. Hence, better to show only the latest edition that they have to work on. i've removed the existing `where` filter for superseded, because `latest_edition` scope will filter-out those.

fixed an unrelated issue in the controller where the pagination params were not being respected.
If we do this, editors looking at a CIP index that contains drafts will have no way of knowing whether there already exists a published version of that page. Is that what we want?