title: Make ActiveAdmin select input display in a tree structure
body: <p>I have a Categories model (HABTM) that has an infinite number of categories and subcategories. I was was rolling my own solution before but I realize now that I am going to have to use Ancestry gem to get this working. When I did my own solution I was able to build a tree because I had a field in my called top_level_category so I knew where to start and a parent_id so I could find my way down the tree. With Ancestry there is just the one string type column to build from. Is there already a solution out there so that I can easy visually select a category and know where where it is in the tree?</p>
  
  <p>I have tried to to activeadmin-sortable-tree gem and can get it to work on the categories index page but not when adding a new category.</p>
  
  <pre><code>sortable tree: true,
             sorting_attribute: :name,
             parent_method: :parent,
             children_method: :children,
             roots_method: :roots
  
  
  form do |f|
          f.inputs "Category", as: :sortable do
            f.input :parent_id, :as =&gt; :select, :collection =&gt; ItemsCategory.all.map {|u| [u.name, u.id]}
            f.input do
              label :name # item content
              actions
            end
            f.input :name
          end
          f.actions
          f.semantic_errors
        end
  </code></pre>