title: Django filter_horizontal filtering
body: <p>I have 2 models related by M2M type of relationship. I use <code>filter_horizontal</code> in the admin for editing my entities.</p>
  
  <p>However, I would like to have a control on what is presented in the left side of the <code>filter_horizontal</code> widget. For example, I would like to filter and show only those entities that match some certain criteria.</p>