Set a starred_at field when an entry is starred.

This date is used to sort starred entries.

Can not use Entry::timestamps method otherwise starred_at will be updated each time entry is updated.
Add an updateStar method into Entry class
A migration script has been added in order to set starred_at field.

src/Wallabag/CoreBundle/Controller/EntryController.php

@@ -333,6 +333,7 @@ class EntryController extends Controller
         $this->checkUserAction($entry);
 
         $entry->toggleStar();
+        $entry->updateStar($entry->isStarred());
         $this->getDoctrine()->getManager()->flush();
 
         $message = 'flashes.entry.notice.entry_unstarred';