adding twig extensions to select how many items per page

* twig anchor: {{ chill_items_per_page(pagination) }}
* select field changed with javascript event listener

src/Bundle/ChillMainBundle/Pagination/PaginatorFactory.php

@@ -61,7 +61,7 @@ class PaginatorFactory
     public function __construct(
           RequestStack $requestStack,
           RouterInterface $router,
-          $itemPerPage = 50
+          $itemPerPage = 20
     ) {
         $this->itemPerPage = $itemPerPage;
         $this->requestStack = $requestStack;