Refactor document list partials

There no material difference between the way announcements and
publications are listed on pages. This consolidates the two partials,
which allows topics#show and topical_events#show to be DRY'd up
Pull out helper for incrementing
Use new partial on topical_events#show
Merge pull request #905 from alphagov/document-list-partial-tweaks

topics#show partial tidy up