Allow CSV upload to specify a field to be used as the key

[waxlamp]

If this is specified, then an existing _key field should fail validation, and the effect should be to validate uniqueness on the specified field, and copy that field over into _key. That is, the existing data field should remain, but a _key field should appear with the same values in it.

[jjnesbitt]

What should we support for downloading in this case then? Would want to delete the _key field and keep the original one, or vice versa?

[waxlamp]

Downloading in this case wouldn't have any special casing--you'd get a CSV with both a _key column and the original data column.