Add a `U2fRegistrations` table/model.

- To hold registrations from U2F devices, and to authenticate them.
- Previously, `User#two_factor_enabled` was aliased to the
  `otp_required_for_login` column on `users`.
- This commit changes things a bit:
    - `User#two_factor_enabled` is not a method anymore
    - `User#two_factor_enabled?` checks both the
      `otp_required_for_login` column, as well as `U2fRegistration`s
    - Change all instances of `User#two_factor_enabled` to
      `User#two_factor_enabled?`
- Add the `u2f` gem, and implement registration/authentication at the
  model level.

1 files changed, 2 insertions, 2 deletions

diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index c28d1ca9e3b..e73b2d08551 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -182,8 +182,8 @@ class ApplicationController < ActionController::Base
   end
 
   def check_2fa_requirement
-    if two_factor_authentication_required? && current_user && !current_user.two_factor_enabled && !skip_two_factor?
-      redirect_to new_profile_two_factor_auth_path
+    if two_factor_authentication_required? && current_user && !current_user.two_factor_enabled? && !skip_two_factor?
+      redirect_to profile_two_factor_auth_path
     end
   end