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.
yes, I had to retake most of the screen shots yesterday for the course-wide discussion topic IDs (not names) fiasco. this one got so corrupted in the process that it killed my locally installed GitHub application. I will start from scratch today to get it corrected