Corrections to guides for 4 Sep release

Items repaired:
- Pin_discussion.png file in wrong directory
- Discussion_filters.png image in discussion_students unreadable, replaced
- updated release date in Research guide change log
- updated release date in Course Author’s guide change log
- updated “History” dates for fields added to research guide

@srpearce if you would please sync to this branch and make html for the course author's guide, that would help us make sure that the two image files are being included correctly (I tested locally, but don't trust what I see any more)

@explorerleslie @gwprice this should resolve both problems with images, and corrects the dates given for the release. Thank you for your efforts investigating!