Fix haskell#783 Don't show button if --quickjump not present

In the TypeScript `init.ts`, makes `quickJump.init()` (which creates the button) conditional on the presence of meta tag `hasQuickJump` with content `"true"`.

In the Haskell, threads the `withQuickJump` flag result through to `headHtml` to create the meta tag, accordingly.

Author: mpilgrem

haddock-api/resources/html/haddock-bundle.min.js

@@ -17,6 +17,9 @@ function onDomReady(callback: () => void) {
 onDomReady(() => {
   document.body.classList.add('js-enabled');
   styleMenu.init();
-  quickJump.init();
+  const hasQJe = document.getElementsByName('hasQuickJump')[0];
+  if (hasQJe) {
+    if ((hasQJe as HTMLMetaElement).content == "true") {
+      quickJump.init();}}
   detailsHelper.init();
 });