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 thought it was confusing to have tests in with non-tests, so I moved the file under the main tests directory. We agreed instead that I will create a tests child directory and move the test file in there.