title: success_url in UpdateView, based on passed value
body: <p>How can I set <code>success_url</code> based on a parameter?<br>
  I really want to go back to where I came from, not some static place.  In pseudo code:</p>
  
  <pre><code>url(r'^entry/(?P&lt;pk&gt;\d+)/edit/(?P&lt;category&gt;\d+)',
      UpdateView.as_view(model=Entry, 
                         template_name='generic_form_popup.html',
                         success_url='/category/%(category)')),
  </code></pre>
  
  <p>Which would mean: edit entry <code>pk</code> and then return to 'category'.  Here an entry can be part of multiple categories.</p>