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.
With my test cleanup this morning this copy is no longer needed. It was really only needed by the random scheme, and these tests do not have access to it anymore.