Update WHATWG links to use HTTPS

Extracted this out of #5649

This commit was created with the following commands:

```
find . -iname "*.webidl" -type f -print0 | xargs -0 sed -i '' 's/http:\(.*\)whatwg.org/https:\1whatwg.org/g'
```

```
find . -iname "*.rs" -type f -print0 | xargs -0 sed -i '' 's/http:\(.*\)whatwg.org/https:\1whatwg.org/g'
```

components/script/dom/characterdata.rs

@@ -117,7 +117,7 @@ impl<'a> CharacterDataMethods for JSRef<'a, CharacterData> {
         Ok(())
     }
 
-    // http://dom.spec.whatwg.org/#dom-childnode-remove
+    // https://dom.spec.whatwg.org/#dom-childnode-remove
     fn Remove(self) {
         let node: JSRef<Node> = NodeCast::from_ref(self);
         node.remove_self();