New chapter on discussion features for all users

@gwprice , @lou-wang , @marcotuts , @mhoeber , @srpearce , @WatsonEmily   : new chapter describing discussion forum features that can be used by both students and staff. Intention is to encourage exploration and identify features that may not be entirely evident in the UI (like the color coding of the dialog bubbles for posts) without over-documenting. 

An HTML version is available here: http://draft-discussions.readthedocs.org/en/latest/running_course/discussions_students.html
The default value in the drop-down used to be "Show All Discussions"; now it is just "All Discussions"