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'd prefer to have a list of second factors from the start. We know it's going to come, it is a requirement that we limit it to one now, but allow multiple in the future. As we discussed, ideally everything is created such that all we have to do is to remove the `assertUserMayAddSecondFactor` (or up the limit there)