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.
This is how the code I refactored was doing it. I was under the impression that it was necessary to reuse the random instance, but that may be unnecessary.