1. Remove an unnecessary (since we're fetching all the records anyway)
`pluck` while fetching U2F registration records.
2. Align "Your device was successfully set up!" section with the "U2F
Devices" table below.
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).
- Move the `TwoFactorAuthsController`'s `new` action to `show`, since
the page is not used to create a single "two factor auth" anymore. We
can have a single 2FA authenticator app, along with any number of U2F
devices, in any combination, so the page will be accessed after the
first "two factor auth" is created.
- Add the `u2f` javascript library, which provides an API to the
browser's U2F implementation.
- Add tests for the JS components
