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/.
The search results don't go directly to specific responses or comments, unfortunately - so all of the items on the right *are* posts, and the user has added a response or comment somewhere in that interaction. I can try to clarify this.