Added 'Next' and 'Previous' links to differential

Summary:
Browsing comments was a bit difficult without the possibllity to jump
between comments. These links will make the browsing easier.

Test Plan:
Tested on multiple diffs that the links were working correctly.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, aran, tuomaspelkonen, epriestley
Differential Revision: 266

src/applications/differential/controller/changesetview/DifferentialChangesetViewController.php

@@ -192,6 +192,8 @@ class DifferentialChangesetViewController extends DifferentialController {
       'whitespace' => $request->getStr('whitespace'),
     ));
 
+    Javelin::initBehavior('differential-comment-jump', array());
+
     $detail = new DifferentialChangesetDetailView();
     $detail->setChangeset($changeset);
     $detail->appendChild($output);
@@ -219,7 +221,8 @@ class DifferentialChangesetViewController extends DifferentialController {
 
     $output =
       '<div class="differential-primary-pane">'.
-        '<div class="differential-review-stage">'.
+        '<div class="differential-review-stage" '.
+          'id="differential-review-stage">'.
           $detail->render().
         '</div>'.
       '</div>';