Don't federate scheduled posts upon claiming

Previously, moving an anonymous post to a blog would instantly federate
the post, regardless of its `created` date. This now respects that value
and doesn't federate the post if its `created` date is in the future.

This is the first part of supporting scheduled, federated posts (ref T567)
but technically fixes #76.

posts.go

@@ -871,7 +871,9 @@ func addPost(app *app, w http.ResponseWriter, r *http.Request) error {
 			if pRes.Code != http.StatusOK {
 				continue
 			}
-			go federatePost(app, pRes.Post, pRes.Post.Collection.ID, false)
+			if !pRes.Post.Created.After(time.Now()) {
+				go federatePost(app, pRes.Post, pRes.Post.Collection.ID, false)
+			}
 		}
 	}
 	return impart.WriteSuccess(w, res, http.StatusOK)