Add a breadcrumbs element

Summary:
Add a basic breadcrumbs element, and implement it in Paste.

This needs some polish but is most of the way there.

Test Plan:
{F26443}
{F26444}
{F26445}

(This element is not visible on devices.)

Reviewers: chad

Reviewed By: chad

CC: aran, btrahan

Maniphest Tasks: T1960

Differential Revision: https://secure.phabricator.com/D4087

src/view/layout/PhabricatorMenuView.php

@@ -49,7 +49,7 @@ final class PhabricatorMenuView extends AphrontView {
   }
 
   public function getItem($key) {
-    return idx($this->map, $key);
+    return idx($this->map, (string)$key);
   }
 
   public function getItems() {