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
I would keep the work posts here instead of thread. Posts have associated responses and comments, and while in some cases it is helpful to explain that the content of the entire post is being discussed, I don't think we need a secondary thread label here. Can follow-up in person next week if we'd like to.