title: Cannot access devise current_admin_user in AcitveAdmin form definition
body: <p>I need to display some form fields in ActiveAdmin form only to specific users.
  But when I try to check user status with this code:</p>
  
  <pre><code>ActiveAdmin.register Store do
    # ...
    form do |f|
      f.inputs "Basic" do
        if current_admin_user.super_admin?
          f.input :admin_user
        end
       # ...
       end
    end
  end
  </code></pre>
  
  <p>I get </p>
  
  <pre><code>undefined local variable or method `current_admin_user' for #&lt;ActiveAdmin::DSL:0xdb8e798&gt;
  </code></pre>
  
  <p>CanCan methods also don't work in the ActiveAdmin form definition.</p>
  
  <p>Generally my question is: how can I manage admin interface display, based on current user type?
  Particularly, how can I get current devise user object from within ActiveAdmin definitions?</p>