Release notes 12-9

@mhoeber @lamagnifica @catong Please review.

@clintonb Since you're release lord (should that be capitalized?) this week, you get a sneak peek at the release notes. You don't have to do a thorough review, but feel free to read through them and let me know if anything seems amiss.
Please add per DOC-1307 and DOC-1325:

*****************
edX Insights
*****************
- The enrollment activity chart, metrics, and report are now available for courses created in Studio before December 4, 2013. Previously, those courses were affected by a change in the method that edX used to track student enrollments. To make enrollment data available in Insights for the affected courses, enrollment history was reconstructed from database tables and user activity logs. EdX Insights now reports enrollment data for the affected courses beginning with November 11, 2013.
- The enrollment activity chart is now presented as a stacked area chart. The filled area
represents the total number of enrolled learners on a particular date. For
courses that offer more than one enrollment option or track, different colors
represent the number of students who are enrolled with each option.