Add ability to create Yubikey, SMS second factor

- [x] Translate translation keys
- [x] Document second factor API in [wiki](https://wiki.surfnet.nl/display/stepup/API+Design)
- [x] Decide on consistent usage of Step-up domain value objects in projection entities
  - [x] `\Surfnet\StepupMiddleware\ApiBundle\Identity\Entity\Identity::$institution` has string type hint.
There's hardly a difference. The URL (route is unavailable, as it's in another application) has to be configured anyway, and this way the nonce can be anywhere in the URL and not as a hard-coded query parameter.