- Implements shortcodes

- Updates newsletter router/sending queue worker to use shortcodes
  replacement class

lib/Newsletter/Shortcodes/Categories/Link.php

@@ -0,0 +1,30 @@
+<?php
+namespace MailPoet\Newsletter\Shortcodes\Categories;
+
+require_once(ABSPATH . 'wp-includes/pluggable.php');
+
+class Link {
+  /*
+    {
+      text: '<%= __('Unsubscribe link') %>',
+      shortcode: 'global:unsubscribe',
+    },
+    {
+      text: '<%= __('Edit subscription page link') %>',
+      shortcode: 'global:manage',
+    },
+    {
+      text: '<%= __('View in browser link') %>',
+      shortcode: 'global:browser',
+    }
+   */
+  static function process($action) {
+    // TODO: implement
+    $actions = array(
+      'unsubscribe' => '',
+      'manage' => '',
+      'browser' => ''
+    );
+    return (isset($actions[$action])) ? $actions[$action] : false;
+  }
+}