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 i've removed the existing `where` filter for superseded, because `latest_edition` scope will filter-out those.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.
from the editor's perspective, given i am on the organisations CIP list, and i want to make a change to the about us page of an organisation, i would want to see the latest edition that i can work on.

i've chosen to show the `latest_edition` because i'm guessing editors would want to see what they want to work on. they can always go to the previous edition when they open the latest edition.

however, we should validate this assumption, for which i had added notes to [the discussion](https://www.agileplannerapp.com/boards/173808/discussions/1332), and was hoping to get responses there which would validate this.

@marksheldonGDS: could you take a look please?