Add a Registration::isNewToHost() method

Problem/Motivation

A lot of places in registration - around host validation especially - use isNew() when actually what they need to consider is whether a registration is new for a host . This hasn't been an issue until now, as registrations haven't been able to change host.

This is useful but not strictly necessary for ✨ Allow admins to change host entity for existing registration Active .

Steps to reproduce

Proposed resolution

Add isNewToHost() to Registration and RegistrationInterface.

Remaining tasks

User interface changes

API changes

Data model changes