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 UI says "Discussion Home" (it's above the name of the course in the right pane). We only use this in a couple of places, at least here in the SFD, and we use "discussions home page" each time. (Saying "go to the *Discussion Home* page feels a little like overkill, so I've just referred to it generically.)