artist urls: add index on url column.

db/migrate/20220403220558_add_url_index_on_artist_urls.rb

@@ -0,0 +1,5 @@
+class AddURLIndexOnArtistURLs < ActiveRecord::Migration[7.0]
+  def change
+    add_index :artist_urls, :url, opclass: :text_pattern_ops
+  end
+end