Make direct-link injection robust to URL fragment changes as the page loads.

Sites such as Medium and EBSCOHost modify the URL fragment as the page
loads, causing direct-link annotation ID fragments to be lost by the
time the app's init code runs.

When a tab's URL changes, extract the '#annotations' fragment and pass
it to the application as configuration info when the app is injected.
If the URL fragment is removed or replaced by client-side JS as the page
loads, the app will still get the original annotation ID that was
captured by the extension prior to the page load completing.

The direct-linked ID is remembered until the sidebar is successfully
injected, at which point it is cleared so that it is not re-used on the
next navigation within the tab.

h/static/scripts/annotator/config.js

@@ -33,6 +33,14 @@ function config(window_) {
     }
   }
 
+  // Extract the direct linked ID from the URL.
+  //
+  // The Chrome extension or proxy may already have provided this config
+  // via a tag injected into the DOM, which avoids the problem where the page's
+  // JS rewrites the URL before Hypothesis loads.
+  //
+  // In environments where the config has not been injected into the DOM,
+  // we try to retrieve it from the URL here.
   var directLinkedID = annotationIDs.extractIDFromURL(window_.location.href);
   if (directLinkedID) {
     options.annotations = directLinkedID;