* Separated models into deferred- and instant-.
* DRY-ed tests around models.

[#81200736]
* VisitorsManipulator now knows which model to manipulate, and can be reused across flows.

[#81200736]
* Instant/Deferred controllers can now generically manipulate visitor data.

[#81200736]
* PrisonerDetailsController now creates a new session without any visitors.

[#81200736]
* Fixed failing tests around missing visitor in the visitor array in the session.

[#81200736]