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.
- [ ] Keep a list of second factors in the agg root
yeah, kinda the same thing :) It's not so much about aggregate root (bad wording on my behalf) as it is about resources. We have an identity resource with related resources which we can get, but they're part of the resource, so should be handled (imo!) by the resource-controller. Yo dawg, I heard you liked resources......