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, 13 insertions, 0 deletions

diff --git a/db/migrate/20160425045124_create_u2f_registrations.rb b/db/migrate/20160425045124_create_u2f_registrations.rb
new file mode 100644
index 00000000000..93bdd9de2eb
--- /dev/null
+++ b/db/migrate/20160425045124_create_u2f_registrations.rb
@@ -0,0 +1,13 @@
+class CreateU2fRegistrations < ActiveRecord::Migration
+  def change
+    create_table :u2f_registrations do |t|
+      t.text :certificate
+      t.string :key_handle, index: true
+      t.string :public_key
+      t.integer :counter
+      t.references :user, index: true, foreign_key: true
+
+      t.timestamps null: false
+    end
+  end
+end