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.
why the max? You already give a default value. It's an internal API, if we supply 0 or negative numbers, we should receive an error (which will propably come in the form of an DoctrineException or something) for being stoopid :tongue: