Rename NewsArticle::Rebuttal to GovernmentResponse

Ideally this would have just been a front-end label change to avoid doing a search re-index, unfortunately the word 'rebuttal' in the search_format_type is generated from one of these front-end labels and so we might as well do a full rename.  There is a data migration to come to remove & reindex all the rebuttals.
Rename rebuttals to government_responses in locale files
Decouple search_format_type from singular_name in NesArticleType

This should mean if we need to rename any of these in the future we don't have to reindex the content as the search_format_type is not dependant on what we want the thing to be displayed on the front-end as.
Remove all published news articles from search, then add again

This so we can put the new search_format_types for government response (née rebuttal) type news.
Merge pull request #393 from alphagov/rebuttals-become-government-responses

Rebuttals become government responses

https://www.pivotaltracker.com/story/show/45594295