Rebuttals become government responses

Rename news article type "Rebuttal" to "Government Response".

What should be a simple frontend label rename is actually a full object rename because the frontend label (NewsArticleType#singular_name) is actually used to generate the search_format_type in the search index.  So we do the full object rename and decouple the frontend labels from the search_format_type.  We also provide a data migration to reindex all the existing news articles.

For: https://www.pivotaltracker.com/story/show/45594295