Add UIDs to datasets without an `id` column

[colleenm]

Currently we put the burden on users to ensure they have a column labeled id in their dataset, but we could do this in like 5 lines of code for a better UX.

Low priority backend feature.

[colleenm]

We should do this -- very minimal code change for a clear UX improvement (nontechnical users are sometimes confused by this column). Doesn't need to block alpha launch but I'd vote to do it for beta