artists: redact version histories of banned artists.

Fix names and urls of banned artists being visible in Google through
artist version pages.

app/models/application_record.rb

@@ -29,6 +29,14 @@ class ApplicationRecord < ActiveRecord::Base
       def visible(user)
         all
       end
+
+      def policy(current_user)
+        Pundit.policy(current_user, self)
+      end
+    end
+
+    def policy(current_user)
+      Pundit.policy(current_user, self)
     end
   end
 

app/models/artist_version.rb

@@ -5,6 +5,14 @@ class ArtistVersion < ApplicationRecord
   belongs_to_updater
   belongs_to :artist
 
+  def self.visible(user)
+    if policy(user).can_view_banned?
+      all
+    else
+      where(artist: Artist.unbanned)
+    end
+  end
+
   module SearchMethods
     def search(params)
       q = search_attributes(params, :id, :created_at, :updated_at, :is_deleted, :is_banned, :name, :group_name, :urls, :other_names, :updater, :artist)