title: django how to display users full name in FilteredSelectMultiple
body: <p>i am trying to use FilteredSelectMultiple widget to display list of users. currently it is displaying only username. I have tried to override the label_from_instance as seen below but it does not seem to work. how can it get to display users full name.</p>
  
  <pre><code>class UserMultipleChoiceField(FilteredSelectMultiple):
      """
      Custom multiple select Feild with full name
      """                                                                                                                                                                 
      def label_from_instance(self, obj):
                return "%s" % (obj.get_full_name())
  
  class TicketForm(forms.Form):
      cc_to  = forms.ModelMultipleChoiceField(queryset=User.objects.filter(is_active=True).order_by('username'), widget=UserMultipleChoiceField("CC TO", is_stacked=True)
  </code></pre>