artists: prevent Google from indexing banned artists.

Prevent search engines from indexing artist pages, wiki pages, and tag
searches for banned artists.

app/logical/post_sets/post.rb

@@ -122,6 +122,7 @@ module PostSets
 
     def hide_from_crawler?
       return true if current_page > 50
+      return true if artist.present? && artist.is_banned?
       return false if query.is_empty_search? || query.is_simple_tag? || query.is_metatag?(:order, :rank)
       true
     end