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.
- the button is still on the same page, just in a different section. Both the one that you mention and the "List enrolled students' profile information" button (only visible for courses with <200 students enrolled) have moved down to the Reports section of the Data Download page. 
- The change is more that clicking this "download" button starts a background task now, so you can navigate away and later return to the Downloads page (you had to stay on the page and wait before). A link to a timestamped CSV file will be added to the table at the bottom of the page for each report you download when report generation is complete.
- the Cohort column appears only if that feature is enabled for the course.