Change cohort assignments and cohort assignment method
After a cohort is selected from the dropdown menu, and the Settings tab is selected, two new fields should appear for cohorts: Cohort Name and Assignment Method. See wireframes here: https://projects.invisionapp.com/share/VX204BTE9#/screens
Minor doc impact to editing settings for cohorts
More substantial impact to existing documentation about assigning students to cohorts and limitations/behavior relating to the default cohort.
Steps to Reproduce
Reason for Variance
User Impact Summary
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