title: Active Admin: Customize only new form
body: <p>I'm using Active Admin to provide an admin to some models. I need to provide a customized new form for one of them, but leave the edit form as the default provided by Active Admin. Here's what I have. It works in that it is giving me the new form I want, but the edit form is also using the new form, which is not what I want:</p>
  
  <pre><code>ActiveAdmin.register Document do
    form :partial =&gt; 'form'
  end
  </code></pre>
  
  <p>I've tried this, but it gives an error that 'new' is an undefined method:</p>
  
  <pre><code>ActiveAdmin.register Document do
    new do
      form :partial =&gt; 'form'
    end
  end
  </code></pre>