title: Django: Admin register decorator for custom admin site class
body: <p>So I want to customize the admin page using the AdminSite class as described in the <a href="https://docs.djangoproject.com/en/1.9/ref/contrib/admin/#customizing-adminsite" rel="nofollow">docs</a>.</p>
  
  <pre><code>from django.contrib.admin import AdminSite
  from .models import MyModel
  
  class MyAdminSite(AdminSite):
      site_header = 'Monty Python administration'
  
  admin_site = MyAdminSite(name='myadmin')
  </code></pre>
  
  <p>But instead of using <code>admin_site.register(MyModel)</code> I would like to use the register decorator like</p>
  
  <pre><code>from django.contrib.admin import ModelAdmin
  
  @admin_site.register(MyModel)
  class MyModelAdmin(ModelAdmin):
      pass
  </code></pre>
  
  <p>However, this only throws the error <code>TypeError: 'NoneType' object is not callable</code>. Is there now register decorator for subclasses of <code>AdminSite</code> in Django yet (1.9) or am I missing something?</p>