Move Stylesheet loading and ownership from the layout task into HTML elements

Stylesheets for `HTMLLinkElement`s are now loaded by the resource task, triggered by the element in question. Stylesheets are owned by the elements they're associated with, which can be `HTMLStyleElement`, `HTMLLinkElement`, and `HTMLMetaElement` (for `<meta name="viewport">).

Additionally, the quirks mode stylesheet (just as the user and user agent stylesheets a couple of commits ago), is implemented as a lazy static, loaded once per process and shared between all documents.

This all has various nice consequences:
 - Stylesheet loading becomes a non-blocking operation.
 - Stylesheets are removed when the element they're associated with is removed from the document.
 - It'll be possible to implement the CSSOM APIs that require direct access to the stylesheets (i.e., ~ all of them).
 - Various subtle correctness issues are fixed.

One piece of interesting follow-up work would be to move parsing of external stylesheets to the resource task, too. Right now, it happens in the link element once loading is complete, so blocks the script task. Moving it to the resource task would probably be fairly straight-forward as it doesn't require access to any external state.

components/script/dom/create.rs

@@ -174,7 +174,7 @@ pub fn create_element(name: QualName, prefix: Option<Atom>,
         atom!("label")      => make!(HTMLLabelElement),
         atom!("legend")     => make!(HTMLLegendElement),
         atom!("li")         => make!(HTMLLIElement),
-        atom!("link")       => make!(HTMLLinkElement),
+        atom!("link")       => make!(HTMLLinkElement, creator),
         // https://html.spec.whatwg.org/multipage/#other-elements,-attributes-and-apis:listing
         atom!("listing")    => make!(HTMLPreElement),
         atom!("main")       => make!(HTMLElement),