Keep url in exists endpoint

- Add migration
- Use md5 instead of sha512 (we don't need security here, just a hash)
- Update tests

src/Wallabag/CoreBundle/Entity/Entry.php

@@ -26,7 +26,7 @@ use Wallabag\UserBundle\Entity\User;
  *     indexes={
  *         @ORM\Index(name="created_at", columns={"created_at"}),
  *         @ORM\Index(name="uid", columns={"uid"}),
- *         @ORM\Index(name="hashedurl", columns={"hashedurl"})
+ *         @ORM\Index(name="hashed_url", columns={"hashed_url"})
  *     }
  * )
  * @ORM\HasLifecycleCallbacks()
@@ -79,7 +79,7 @@ class Entry
     /**
      * @var string
      *
-     * @ORM\Column(name="hashedurl", type="text", nullable=true)
+     * @ORM\Column(name="hashed_url", type="string", length=32, nullable=true)
      */
     private $hashedUrl;