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 didn't write this code and couldn't think of a better string for this (and needed something to pass the quality check). :-) The two nested functions are very subtly different...