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/.
I'm not sure what you're seeing here. For courses that have the cohort feature implemented, you see a "visibility" indicator. It reads "This post is visible to everyone" or "This post is visible only to {name of cohort}." I don't know of a setting that makes a post visible only to you and to course staff. 
If that's what you mean, I'd drop "privacy" for "visibility", and reorganize to make this less prominent (as it is an optional feature)