Filter new from the sub-topic 'A-Z' list

Story: https://www.pivotaltracker.com/story/show/82281582

Exclude news articles from the 'A-Z' list, mitigating the user
impact of a department tagging loosely related news content to a
sub-topic when there is no curated list.
Simplify `SectorPresenter#filtered_contents` logic
Add world_location_news_article to excluded formats
Merge pull request #16 from alphagov/filter-new-and-announcements

Filter news from the sub-topic 'A-Z' list

https://www.pivotaltracker.com/story/show/82281582