TNL-328 Cohort/server display auto

[TNL-180](https://openedx.atlassian.net/browse/TNL-180)

@jimabramson In reading your comment again, "it's better for us to use factories.. to *set up the data* with which we're testing the API.", are you Ok with me using `CohortFactory` to create the test cohorts, but then using the existing helper methods in `cohort.py` (such as `get_cohort_by_name` and `get_cohort_by_id`) to verify the tests?  These can be viewed as low-level helper methods to be called from both test and production code.