title: Add a custom action button in active_admin input form
body: <p>Iam working on active_admin form. my requirement is to add a button beside a input box to create new project
  How to add a action item inside a form ?</p>
  
  <pre><code>form do |f|
      f.semantic_errors *f.object.errors.keys
      f.inputs "Details" do
        f.input :task
        f.input :project
        f.input :members, :input_html =&gt; { :class =&gt; "chosen-input" }, :label =&gt; 'Assigned To'
        f.actions
      end
   end
  </code></pre>
  
  <p>i want to add a new project button beside input field project</p>