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.
In Java, this would be static meaning that the binding itself doesn't change, but it doesn't govern the immutability of the object itself. I hadn't realized that the Python convention is actually stricter than that. I'll switch it.