Reorg of cohorts and discussion topics

Pulled the reorg of cohort and discussion topics out of previous branch DOC-1077 so that it is no longer dependent on that open source PR. Topics about setting up and managing discussion now separated from the cohort configuration section.
@mhoeber @lamagnifica @srpearce @explorerleslie please give thumbs if you're OK with the reorg.
Would like to create new doc for cohorted courseware on top of this file organization.
Oops I goofed -- I had to break apart my previous branch and missed moving that out of the "new" version of the cohort_config.rst. Will commit with that further change. Thanks for catching.