set referrer in window.load_url

- this conforms to follow-hyperlinks spec step 13
- this conforms to window-open spec step 14.3
- replace uses of `referrer_url` with `referrer`
- in Request class, change "no-referrer" to ""
- set websocket fetch referrer to "no-referrer"

components/script/dom/request.rs

@@ -554,7 +554,7 @@ impl RequestMethods for Request {
     fn Referrer(&self) -> USVString {
         let r = self.request.borrow();
         USVString(match r.referrer {
-            NetTraitsRequestReferrer::NoReferrer => String::from("no-referrer"),
+            NetTraitsRequestReferrer::NoReferrer => String::from(""),
             NetTraitsRequestReferrer::Client => String::from("about:client"),
             NetTraitsRequestReferrer::ReferrerUrl(ref u) => {
                 let u_c = u.clone();