Soft delete feature for comment [NO MERGE]

I agree: delete and restore seem like the only operations we need -- the 410 Gone from a GET (and HEAD?) should be enough to know if it exists.  Having /fcr:restore and /fcr:delete seem reasonable enough to me.  The other alternative that comes to mind would be to have some kind of delete/restore token with a POST request, but that's probably more complicated/confusing.