Make SigningService in the DB immutable
It was determined there that making the SigningService model itself immutable is a good thing for these reason:
- Content signers should be able to leave their existing publications pointing to the old SigningService, otherwise their data will become invalidated.
- Content signers should have to take explicit action when a public key is rotated, especially so they can know to also update clients.
Make it immutable, the data contained in a SigningService, or its subclass should not be modified once saved.