If category is deleted, all articles are silently unpublished

[echarlie]

If a category is deleted, all pages in that category are "unpublished", i.e. are not visible on any of the news feeds. Pages are available if you go directly to the slug, and "Filed Under " (NULL), however the edit page lists them as filed under the first category, and as published.

We should either

1. Unpublish articles (expected behaviour) and complain loudly when a category with articles in it is deleted OR
2. Do what appears to happen, which is to shove all articles into a "default" category so that they're still visible in feeds.