Use created_at as default sort

Imported content use the real creation date. If we sort by id, it'll
display contents in reverse order.

src/Wallabag/CoreBundle/Repository/EntryRepository.php

@@ -22,7 +22,7 @@ class EntryRepository extends EntityRepository
         return $this->createQueryBuilder('e')
             ->leftJoin('e.user', 'u')
             ->andWhere('u.id = :userId')->setParameter('userId', $userId)
-            ->orderBy('e.id', 'desc')
+            ->orderBy('e.createdAt', 'desc')
         ;
     }