title: FOSUserBundle: Remove unique index for emailCanonical
body: <p>I am trying to remove the unique index on emailCanonical, so that multiple users can share the same email address. However, I do not want to edit FOS/UserBundle/Resources/config/doctrine/User.orm.xml directly as any updates to the bundle itself will remove the change. Is there any way I can override the emailCanonical field in my own bundle, while extending the base user (FOS/UserBundle/Model/User.php)</p>
  
  <pre><code>use FOS\UserBundle\Entity\User as BaseUser;
  use Doctrine\ORM\Mapping as ORM;
  use Foo\BarBundle\Constant\SecurityConstant;
  
  class User extends BaseUser {
      protected $id;
  ...
  }
  </code></pre>
  
  <p>Thanks in advance!</p>