Improve usability of assigning a unique column to a Track table

[karrla]

Story

To merge two tables you have to have assigned a unique column.
To update a table from Google sheets (if you don't want to overwrite the table) you have to assign a unique column.

Assigning a unique column has to be done by editing the table, it is not very clear to users that this is required and how to assign a unique column.

Acceptance Criteria

UX

@joy-humanitec and/or @GuruMuni
We need some UX assistance with this issue