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

webroot/rsrc/js/application/differential/behavior-comment-jump.js

@@ -0,0 +1,29 @@
+/**
+ * @provides javelin-behavior-differential-comment-jump
+ * @requires javelin-behavior
+ *           javelin-util
+ *           javelin-dom
+ */
+
+JX.behavior('differential-comment-jump', function(config) {
+  function handle_jump(offset) {
+    return (function(e) {
+      var parent = JX.$('differential-review-stage');
+      var clicked = e.getNode('differential-inline-comment');
+      var inlines = JX.DOM.scry(parent, 'div', 'differential-inline-comment');
+      var jumpto = null;
+
+      for (var ii = 0; ii < inlines.length; ii++) {
+        if (inlines[ii] == clicked) {
+          jumpto = inlines[(ii + offset + inlines.length) % inlines.length];
+          break;
+        }
+      }
+      JX.DOM.scrollTo(jumpto);
+      e.kill();
+    });
+  }
+
+  JX.Stratcom.listen('click', 'differential-inline-prev', handle_jump(-1));
+  JX.Stratcom.listen('click', 'differential-inline-next', handle_jump(+1));
+});