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.
As a note, Collections.reverseOrder can be avoided if dateComparator has the line "return o2.getCreatedDate().compareTo(o1.createdDate()). But probably better to be explicit about the descendingness.