Add all domain functionality, read models and APIs for Self-Service functionality

- [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.
- [x] Keep a list of second factors in the agg root
- [x] Refactor projections to have one (immutable) object per state.
- [x] Move controller actions with identity resource root into identity controller.
- [x] Store the identity's preferred locale in the event in those cases where it is needed for sending e-mail
- [x] Revert to stable Symfony2 version.