Store group sets in their own table

[marcospri]

Groups set (also known as group categories in some LMSes) are "group a groups". They are now stored as JSON in the course row.

Store them in their own table for easier querying.

See: https://docs.google.com/document/d/1AuP9qDgvzOoaVdsla--jOWaMyVSj1XwY5CWRQPTDX6o/edit?tab=t.0#heading=h.1cay7rutjflp