record column used as -i / feature ID it in the`user` block of the space object

[burritojustice]

If the user uploads a CSV to a space and uses -i to select a column as the feature ID, record it in the user object -- maybe something like csv_feature_id: column_name.

The next time a user uploads to that space, if they don't use the same column for -i, we may want to issue a warning.