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 doesn't much matter as all the mock schemes behave the same way, but to make it seem clearer I've made it pick a scheme out of the extensions.