Add a filter by user and due date + minor changes

models/user.php

@@ -22,6 +22,13 @@ class User extends Base
      */
     const TABLE = 'users';
 
+    /**
+     * Id used for everbody (filtering)
+     *
+     * @var integer
+     */
+    const EVERYBODY_ID = -1;
+
     /**
      * Get a specific user by id
      *