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.
Are these Ids _within the domain_? Can they be used as such? If so, then there should not be a separate SecondFactorId. If not, then they may not implement the Id interface. In my mind, the latter is the case.