When navigating to a new page, it should be scrolled to the top + navigating to a fragment on another page broken in Firefox

+++ b/js/refreshless.js
@@ -270,10 +272,11 @@
-          location.assign('#' + target.fragment);
+          var element = document.getElementById(target.fragment);
+          if (element) {
+            element.scrollIntoView();
+          }

This change is necessary due to different behaviors in different browsers:

• Google Chrome doesn't create a new history entry for location.assign() (perhaps intentionally, perhaps due to a race condition, who knows)
• But Firefox does

Therefore I had to resort to manual scrolling for this. Fortunately we have a good API for just that.

Now that we have manual scrolling in place, and it turns out to be trivial, we might as well do that when navigating to a fragment on the current page too, because that'd allow us to remove our onhashchange event handler. Let's do that in a follow-up.