title: Add custom button to edit page of sonata admin bundle
body: <p>As you know, sonata admin bundle comes with three buttons in edit page which are "Add new, update and delete". I can remove delete button with this:</p>
  
  <pre><code>protected function configureRoutes(RouteCollection $collection)
  {
      $collection
          -&gt;remove('delete')
      ;
  
  }
  </code></pre>
  
  <p>But I want to also add "Send message to User" button in edit of UserAdmin. How can I do this? I can't find any documentation about that in sonata docs.</p>