Replace the implicit unique index with an explicit one.
Replace the implicit unique index with an explicit one.

This is functionally identical, but because we rely on this index we should make it explicit and named.

(for example, if the unique constraint was dropped in the future we would still want the column[s] indexed.)

alex-ip/agdc,ama-jharrison/agdc,ama-jharrison/agdc,alex-ip/agdc