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. I didn't know much about that - just saw "this post is visible to everyone" and made some assumptions. I've looked at a few different courses, though, and the "This post is visible to everyone" appears in courses that don't have cohorts (including my toy course, which is where I got this screen shot). I think I'll just take out this information for now; it's pretty self-explanatory.