DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
1. Display a list of U2F devices on the `two_factor_auth` page. 2. Allow deleting individual U2F devices. 3. Allow setting a (optional) name for a device (during registration).