add primary key to (possibly) enable drill-down

[mroswell]

After importing a small CSV, I used datasette enrichments to generate a full_address field, and an opencage_json field. The problem is that I can't see the full opencage_json data. I thought I could click on the rowid to drill into detail, but it is not linked. I'm guessing that that's because there is no primary key. If that's the case, it might be helpful to offer settings in datasette-upload-csvs, so that the primary key defaults to rowid unless another column is selected.