Draft of release notes for 10-7

@lamagnifica @mhoeber @explorerleslie Please review. (Leslie - in particular, can you review the bullets that have to do with discussions?) Built version at http://draft-107-release-notes.readthedocs.org/en/latest/10-07-2014.html.
I would say "private discussion cohorts", as opposed to just "cohorts". There are so many different understandings of what "cohorts" is that I've been trying to message this really clearly.