Added optional two-factor authentication

db/migrate/20170127165745_add_devise_two_factor_to_users.rb

@@ -0,0 +1,9 @@
+class AddDeviseTwoFactorToUsers < ActiveRecord::Migration[5.0]
+  def change
+    add_column :users, :encrypted_otp_secret, :string
+    add_column :users, :encrypted_otp_secret_iv, :string
+    add_column :users, :encrypted_otp_secret_salt, :string
+    add_column :users, :consumed_timestep, :integer
+    add_column :users, :otp_required_for_login, :boolean
+  end
+end