PERF: Add missing index on `user_id` for `user_second_factors` table.

app/models/user_second_factor.rb

@@ -31,3 +31,7 @@ end
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
 #
+# Indexes
+#
+#  index_user_second_factors_on_user_id  (user_id)
+#

db/migrate/20180716062012_add_index_user_id_on_user_second_factors.rb

@@ -0,0 +1,5 @@
+class AddIndexUserIdOnUserSecondFactors < ActiveRecord::Migration[5.2]
+  def change
+    add_index :user_second_factors, :user_id
+  end
+end