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.
I added the JmsTranslationBundle to extract and translate validation messages etc. As long as we don't add an API whose URL starts with `css/`, `images/` and `js/` we should be safe. What is your preferred course of action?