title: Date after another date in symfony form
body: <p>I have a form that contain 2 dates: start date(datedebut) and end date(datefin). 
   I want the end date to be always after the start date. How can i do that?</p>
  
  <p>My form type:</p>
  
  <pre><code>class ReservationType extends AbstractType
  {
      /**
       * {@inheritdoc}
       */
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder
              -&gt;add('datedebut',DateType::class,array(
                  'widget' =&gt; 'choice',
                  'years' =&gt; range(date('Y'), date('Y')+20),
              ))
              -&gt;add('datefin',DateType::class,array(
                  'widget' =&gt; 'choice',
                  'years' =&gt; range(date('Y'), date('Y')+20),
              ))
              -&gt;add('nbplaces')
          ;
      }
  
      /**
       * {@inheritdoc}
       */
      public function configureOptions(OptionsResolver $resolver)
      {
          $resolver-&gt;setDefaults(array(
              'data_class' =&gt; 'Bridge\TravelBundle\Entity\Reservation'
          ));
      }
  
      /**
       * {@inheritdoc}
       */
      public function getBlockPrefix()
      {
          return 'Bridge_TravelBundle_Reservation';
      }
  }
  </code></pre>