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
How about:

* A match or close match to your text in any post.....
* A similar value in any post... (keep as is)
* Any user names that are an exact match to your text. In addition to the above results, a "Show posts by {username} option...."

I'd love it if we make this point, since we've received specific feedback in the past about what course teams want search to do. I'm willing to drop the hair-splitting if this is turning the section into something cumbersome, though.