Add multi_birth_id field to the participant_person_link model. #3977.
Ignore multi_birth_id for participant_person_link for operational enumerators except MDES 3.1 and 3.2. #3977.
Merge pull request #150 from NUBIC/change_3977

Add multi_birth_id to the participant_person_links model and include in the appropriate enumerator. Closes #3977.