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.
We use it in opaque_keys: https://github.com/edx/opaque-keys/blob/master/opaque_keys/__init__.py#L239