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, 8 insertions, 0 deletions

diff --git a/spec/factories/u2f_registrations.rb b/spec/factories/u2f_registrations.rb
new file mode 100644
index 00000000000..df92b079581
--- /dev/null
+++ b/spec/factories/u2f_registrations.rb
@@ -0,0 +1,8 @@
+FactoryGirl.define do
+  factory :u2f_registration do
+    certificate { FFaker::BaconIpsum.characters(728) }
+    key_handle { FFaker::BaconIpsum.characters(86) }
+    public_key { FFaker::BaconIpsum.characters(88) }
+    counter 0
+  end
+end