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/models/artist_version.rb

@@ -52,11 +52,11 @@ class ArtistVersion < ApplicationRecord
   end
 
   def was_deleted
-    !is_active && previous.is_active
+    is_deleted && !previous.is_deleted
   end
 
   def was_undeleted
-    is_active && !previous.is_active
+    !is_deleted && previous.is_deleted
   end
 
   def was_banned