Create auto cohorts when list_cohorts is called.

TNL-186

@andy-armstrong @jimabramson Please review.
The other reason that I dislike the current code is that it makes assumptions about the ids that will be assigned to the auto groups (namely that they are sequential and come immediately after the one assigned to cohort3). That problem could also be avoided by not asserting anything about the ids of the returned cohorts, which I think would be acceptable in this case.