TNL-328 Cohort/server display auto

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

I'm going to:
1. move the existing `CohortFactory` class from `test_views.py` to `helpers.py`.
2. replace `cohorts.add_cohort` calls in the test ---> `CohortFactory.create`

3. create a `get_cohort_by_name` helper method in `helpers.py`.
4. replace `cohorts.get_cohort_by_name` calls in the tests -> get_cohort_by_name in helpers.py.