/ip_addresses: add missing created_at, uploader_ip_addr indexes.

Add some missing indexes used by the /ip_addresses page.

db/migrate/20191111024520_index_tables_on_created_at.rb

@@ -0,0 +1,8 @@
+class IndexTablesOnCreatedAt < ActiveRecord::Migration[6.0]
+  def change
+    add_index :artist_commentary_versions, :created_at
+    add_index :users, :created_at
+    add_index :comments, :created_at
+    add_index :posts, :uploader_ip_addr
+  end
+end