Add support in the core data model for free-text cause of death.
- Add a "person.cause_of_death_non_coded" field
- In PersonValidator, ensure that only one of Person.causeOfDeath and Person.causeOfDeathNonCoded can be set
- Ensure there's a ticket to add support for this to REST also
This ticket has been suggested through a talk post https://talk.openmrs.org/t/mark-patient-as-dead/6279/23.