Reset `otp_grace_period_started_at` after disabling 2FA

Prior, if the user enabled 2FA, then disabled it and came back some time
after the grace period expired, they would be forced to enable 2FA
immediately.

app/models/user.rb

@@ -366,6 +366,7 @@ class User < ActiveRecord::Base
       encrypted_otp_secret:        nil,
       encrypted_otp_secret_iv:     nil,
       encrypted_otp_secret_salt:   nil,
+      otp_grace_period_started_at: nil,
       otp_backup_codes:            nil
     )
   end

spec/factories.rb

@@ -32,6 +32,7 @@ FactoryGirl.define do
       before(:create) do |user|
         user.two_factor_enabled = true
         user.otp_secret = User.generate_otp_secret(32)
+        user.otp_grace_period_started_at = Time.now
         user.generate_otp_backup_codes!
       end
     end

spec/models/user_spec.rb

@@ -268,6 +268,7 @@ describe User, models: true do
       expect(user).to be_two_factor_enabled
       expect(user.encrypted_otp_secret).not_to be_nil
       expect(user.otp_backup_codes).not_to be_nil
+      expect(user.otp_grace_period_started_at).not_to be_nil
 
       user.disable_two_factor!
 
@@ -276,6 +277,7 @@ describe User, models: true do
       expect(user.encrypted_otp_secret_iv).to be_nil
       expect(user.encrypted_otp_secret_salt).to be_nil
       expect(user.otp_backup_codes).to be_nil
+      expect(user.otp_grace_period_started_at).to be_nil
     end
   end