Automatically merge exact duplicates

[joeroe]

Exactly duplicated records should be automatically merged after save.

The oldest non-superseded record should be the one that remains, to avoid unnecessary turnover of public-facing IDs (e.g. if we have a long-standing record X, with its ID linked externally, a user creating an exact duplicate Y should not cause X to be marked as superseded). This is different to the merging of non-exact duplicates (#308), which creates a new record.

This should be sufficient to eliminate them all, but just to be safe, and to deal with the large backlog of duplicates we have now, we could also consider a task that periodically or on a trigger runs the same check on all records.