title: Scopes as filters in rails_admin
body: <p>I am using rails_admin in my app.  I have some scopes on my models, following is an example:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
  
    scope :unconfirmed, where('confirmed_at IS NULL')
  
  end
  </code></pre>
  
  <p>Is it possible in rails_admin to get access to those scope as a filter? Like you can in active admin.  Like adding a button somewhere on in the users section.</p>
  
  <p>Thanks</p>