TNL-328 Cohort/server display auto

TNL-328

**NOTE:** 2nd commit only.  The first commit is cherry-picked from a different PR that is currently under review.

@andy-armstrong Please review.  I've created the `CohortRuleType` enum based on our discussion.  I chose to separate "default" and "random" at this layer since they are technically different "rules".  "auto" seemed too general a term, since any future rules we support (e.g., "email-based") would technically also be "auto".  The UI layer can choose whether or not to present them as combined categories.
@cahrens @dan-f  @gwprice FYI.
@jimabramson please review.
The default cohort is created if no auto cohorts exist. This first sentence reads differently.

Suggested: "A 'default cohort' is an auto-cohort that is automatically created for a course if no auto-cohorts have been specified.