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/.
Oh, OK. How about this? (I'm not sure what to do about "if necessary" - I didn't have it at first, but I thought "course staff can find this information" sounded threatening too. I added "if necessary" to indicate that course staff won't usually look for it. Any ideas?)

In a few courses, you can add posts, responses, and comments anonymously. If
this is the case, you'll see a **Post Anonymously** check box under the field
where you enter your text. When you post anonymously, other students won't be
able to see your username, but course staff can find this information if
necessary.