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.
Add as 2nd sentence something like "When this optional feature is enabled for a course, the course team defines smaller communities of students within the larger, course-wide community."