Add recovery code support for two-factor auth (#1773)

* Add recovery code support for two-factor auth

When users enable two-factor auth, the app now generates ten
single-use recovery codes. Users are encouraged to print the codes
and store them in a safe place.

The two-factor prompt during login now accepts both OTP codes and
recovery codes.

The two-factor settings UI allows users to regenerated lost
recovery codes. Users who have set up two-factor auth prior to
this feature being added can use it to generate recovery codes
for the first time.

Fixes #563 and fixes #987

* Set OTP_SECRET in test enviroment

* add missing .html to view file names

spec/models/user_spec.rb

@@ -1,6 +1,9 @@
 require 'rails_helper'
+require 'devise_two_factor/spec_helpers'
 
 RSpec.describe User, type: :model do
+  it_behaves_like 'two_factor_backupable'
+
   describe 'validations' do
     it 'is invalid without an account' do
       user = Fabricate.build(:user, account: nil)