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
If that's the case, it would be helpful to clarify that in the text - e.g., "click the **Discussion** tab at the top of the screen, and then select..." Even if we get rid of "at the top of the screen," it would be helpful to keep the word "tab."