Fix #4017: Artist tag in upload page should account for aliases

Disallow creating artist entries for aliased tags. Add a fix script to
move existing artist entries for tags that have been aliased.

script/fixes/098_fix_aliased_artists.rb

@@ -0,0 +1,29 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+# Fix artist entries where the tag was aliased but the URLs weren't moved to
+# the new artist entry.
+with_confirmation do
+  CurrentUser.scoped(User.system, "127.0.0.1") do
+    aliased_artists = Artist.joins(:tag_alias).where.associated(:urls).distinct
+
+    aliased_artists.each do |artist|
+      next if artist.tag.category_name != "Artist"
+      next if artist.tag_alias.consequent_tag.category_name != "Artist"
+
+      old_name = artist.name
+      new_name = artist.tag_alias.consequent_name
+
+      artist.update_attribute(:is_deleted, false) # undelete first so the tag move works
+      TagMover.new(old_name, new_name).move_artist!
+      added_urls = (artist.urls - Artist.find_by_name(new_name).urls).map(&:url)
+
+      puts({
+        old_name: old_name,
+        new_name: new_name,
+        added_urls: added_urls,
+      }.to_json)
+    end
+  end
+end