artists: rename is_active flag to is_deleted.

Rename is_active to is_deleted. This is for better consistency with
other models, and to reduce confusion over what "active" means for
artists. Sometimes users think active is for whether the artist is
actively producing work.

app/logical/post_sets/post.rb

@@ -37,7 +37,7 @@ module PostSets
 
     def artist
       return nil unless tag.present? && tag.category == Tag.categories.artist
-      return nil unless tag.artist.present? && tag.artist.is_active?
+      return nil unless tag.artist.present? && !tag.artist.is_deleted?
       tag.artist
     end