Sylvia/docs/sfd discussions

@mhoeber @lamagnifica @catong This is finally ready for review. I accidentally added some extra images, so please ignore the ones that don't apply (I'll take them out before I merge this). I tried to create a built version but RTD inexplicably created the course staff release notes page (grr); will try again tomorrow.

Update: RTD is working! See http://draft-sfd-discussions.readthedocs.org/en/latest/.
OK, here's what I came up with:

* A list of all the posts that contain a contribution from the participant,
  whether the participant created the original post or contributed a response or
  comment to a different post. The list of posts includes the title of the post
  and the username of the participant who created the post. To see the comments
  or responses on the post, click **View Discussion** below the post to open the
  discussion on the **Discussion** page.