Ahodges/doc/cohorts

@explorerleslie , @nasthagiri , @andy-armstrong , @gwprice , @jimabramson , @dan-f , @cahrens, @srpearce  , @WatsonEmily , @mhoeber 
The cohorts/cohort_config and cohorts/cohorts_overview files and images are pretty solid. Questions occasionally in comments.
I'll work on the section describing moderating discussions in a cohorted universe next.
https://openedx.atlassian.net/browse/TNL-160, which was implemented in https://github.com/edx/edx-platform/pull/5189. The name of the default group is "Default Group".