Add base support for cohorted group configurations

TNL-649

Introduced a new scheme field to user partitions that controls how a user gets assigned
to one of the partitions groups. The default scheme is random, but the next step will be
to introduce a new scheme that will assign the user based upon the cohort they belong to.
I was thinking that the default scheme was just for backward compatibility. I can rename the field to make that clearer, or do you think it is better to have a default?