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/applications/directory/controller/PhabricatorDirectoryMainController.php

@@ -496,7 +496,7 @@ final class PhabricatorDirectoryMainController
       'Share Files');
     $nav_buttons[] = array(
       'Create Paste',
-      '/paste/',
+      '/paste/create/',
       'create-paste',
       'Share Text');