topics#show partial tidy up

Pull request with tweaks for: https://github.com/alphagov/whitehall/pull/899 

This DRYs up topics#show and topical_events#show by consolidating some partials that were materially the same. There is still more that could be done (e.g. making topics and topical events more consistent and looking at the similarities of document list rendering on other parts of the site), but this is a step in the right direction.