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.
It will throw a missing field error. I suppose I should have made it throw a TypeError as it does for the other fields. I'll make that change.