deprecate facing_back
Per the wiki, facing_back is just looking_back but with eyes closed, but that could just be looking_back closed_eyes or something.
It also seems to be a large source for mistagging, as many posts tagged as facing_back don't even have closed eyes, or is from_behind rather than looking_back.
The name "facing back" doesn't really seem to imply that the eyes need to be closed.