Add machine-readable pagination using HTML4.01 <link rel="next">, etc.

These extra `<link>` elements only appear on pages where pagination
makes sense. They trigger functionality in some user agents, such as
Opera's Navigation Bar for more easily navigating forward and backwards
across a paged set of notices, messages, or group lists, etc.

actions/outbox.php

@@ -62,6 +62,28 @@ class OutboxAction extends MailboxAction
         }
     }
 
+    /**
+     * Output document relationship links
+     *
+     * @return void
+     */
+    function showRelationshipLinks()
+    {
+        // Machine-readable pagination
+        if ($this->page > 1) {
+            $this->element('link', array('rel' => 'next',
+                                         'href' => common_local_url('outbox',
+                                                                    array('nickname' => $this->user->nickname,
+                                                                          'page' => $this->page - 1)),
+                                         'title' => _('Next Messages')));
+        }
+        $this->element('link', array('rel' => 'prev',
+                                     'href' => common_local_url('outbox',
+                                                                array('nickname' => $this->user->nickname,
+                                                                      'page' => $this->page + 1)),
+                                     'title' => _('Previous Messages')));
+    }
+
     /**
      * retrieve the messages for this user and this page
      *