Warn user when checking a record will cause it to be merged

[joeroe]

Users can update a record in such a way that it becomes an exact duplicate of another and, on save, will be merged into that one (#309).

This is an intended and necessary part of our strategy for data cleaning, but in some cases it could be unexpected and is harder to undo than other changes to records.

The UI should warn the user if the changes they are about to save will cause the record to be merged into another record. The basic behaviour should be some sort of validation on update, but one which the user can confirm they want to override. We can enhance this with an async check before the record is saved, in which case we should assume the user wants to override if they save the changes.