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 suggest taking "and course authors" out of this sentence, and adding a second sentence about how "Discussion admins, moderators, and community TAs can edit the topic of any post" (if that's actually true)