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'd rather keep it conditional on if the split_test module is enabled at this point. Once we implement the UI for creating learner sequences on the Group Configurations page, we can show the Group Configurations page if split_test is enabled or if cohort configuration is ON.