Local news for local people

WorldLocationNewsArticle is a new kind of Announcement that shares some functionality with NewsArticle (see the introduction of Newsesque base class).  The content of them is pushed through rummager, but by default it is excluded from search results.  You can include them in the results with a checkbox in the filter form ("include location-specific news").  The presence of this check box is controlled by a feature flag which is on for dev / preview, but we've turned it off in staging / production.

Things that require attention:

1. the story asks for world location news to appear in a latest updates box on the worldwide priorities page.  I've done this, but it obviously needs styling and I don't even know if it's in the right place on the page.  It also has an atom feed url (currently just pointing at the page itself, I don't know if we need one?)

2. There's no way to see just world location news on the announcements index.  We could add it as a separate announcement type in the dropdown, or make the existing "news stories" option reveal NewsArticle and WorldLocationNewsArticle instances (respecting the "Include location-specific news" toggle).  The problem being that WorldLocationNewsArticle instances don't have topics or organisations, so as soon as you choose to see WorldLocationNewsArticles and filter by topic or org you will get no results.  Maybe this needs some more thought if we need to do it.

https://www.pivotaltracker.com/story/show/33557615