Keep history of admin unit reviews

Allow multiple entries of admin unit reviews: no more unique constraint on (expert id, disease group id, admin unit gaul code) triplet, and changed_date is now created_date

List of all admin unit reviews for one disease group is then filtered to only return the most recent review of an admin unit by each expert.
Is it worth replacing these with a (expert_id, disease_group_id, gaul_code, created_date) unique constraint? Just for catching coding errors early.