title: Django custom admin site doesn't work in Model view
body: <p>I implemented a custom admin site with a custom header and when viewing the "Home" and "App" views, everything is going well. However, when I inspect the "Model" instances, the header goes back to the default.</p>
  
  <p>How do I fix this? Have I done something wrong or is it reasonable to consider this a bug in Django?</p>
  
  <h1>Problem Demonstration</h1>
  
  <p>App View:
  <a href="https://i.stack.imgur.com/TDNwc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TDNwc.png" alt="good header"></a></p>
  
  <p>Model View:
  <a href="https://i.stack.imgur.com/HdlJ5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HdlJ5.png" alt="bad header"></a></p>
  
  <h1>MCVE</h1>
  
  <p>Make a new project and app, and in a file <code>sites.py</code> in the app:</p>
  
  <pre><code>from django.contrib import admin
  
  
  class CustomSite(admin.AdminSite):
      site_header = 'Custom Header'
      site_title = 'Custom Title'
  
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
          self._registry.update(admin.site._registry)
  
  
  custom_site = CustomSite('custom_admin')
  </code></pre>
  
  <p>Then, in your project URL configuration <code>urls.py</code>:</p>
  
  <pre><code>from django.contrib import admin
  from django.urls import path
  
  from adminfail.sites import custom_site
  
  urlpatterns = [
      path('custom_admin/', custom_site.urls),
  ]
  </code></pre>
  
  <p>Also, add the app to your <code>INSTALLED_APPS</code> in <code>settings.py</code>.</p>
  
  <p>Then, if you create a superuser from the command line and run the app, you'll see that the custom header goes back to the default when inspecting Model instances.</p>