title: Django Admin list_display - How to list a dictionary value from a JSON-Field with list_display
body: <p>I want to display a specific Value from a JSON-Field in Django Admin list_display.</p>
  
  <pre><code>models.py:
  class Foo(models.Model):
  client = models.ForeignKey(Client, verbose_name="Kunde")
  
  description = models.CharField(verbose_name=u'Titel', max_length=100)
  description_file = models.FileField(verbose_name="Vorschau", upload_to='postings', blank=True, help_text=u'Vorschau/Endfassung')
  accepted_by_client = models.BooleanField(verbose_name="abgenommen", help_text=u"")
  needs_correction = models.BooleanField(verbose_name=u"Korrektur nötig", help_text=u"")
  active = models.BooleanField(verbose_name="Publiziert", help_text=u"")
  ctime = models.DateTimeField(verbose_name="Angelegt am", blank=True, null=True)
  valid_until = models.DateField(verbose_name=u"gültig bis", blank=True, null=True, default = lambda: datetime.datetime.now()+datetime.timedelta(days=28) )
  valid_from = models.DateField(verbose_name=u"gültig ab", blank=True, default=datetime.datetime.now ) 
  link = models.CharField(verbose_name="Formular", max_length=255, blank=True, help_text="generierter Link zum Formular")
  admin_notes = models.TextField(u'Notizen für Mitarbeiter', blank=True, help_text=u"")
  client_notes = models.TextField(verbose_name=u"Notizen für Kunde", blank=True, help_text=u"")
  
  context = JsonField()
  
  def __unicode__(self):
      return self.client.company_name + ' : ' + str(self.id)
  
  admin.py:
  
  class FooAdmin(admin.ModelAdmin):
      #list view
      def apis(self, object):
          data = Foo.context['bar']
          return data
  
      list_display = (
          'id', 
          'client',
          'description', 
          'active', 'ctime', 
          'valid_until',
          'valid_from',
          'clickable_preview_link',
          'clickable_editor_link',
          'apis'
      )
  
  admin.site.register(Foo, FooAdmin)
  </code></pre>
  
  <p>My Problem is, I can't access the single values from the JSON-Field "context". I'm only receiving a empty column.</p>
  
  <p>How can I do this?</p>
  
  <p>Thanks in advance!</p>