title: Django custom user model in admin, relation "auth_user" does not exist
body: <p>I have a custom user model as below:</p>
  
  <pre><code>class User(AbstractUser):
      subscribe_newsletters = models.BooleanField(default=True)
      old_id = models.IntegerField(null=True, blank=True)
      old_source = models.CharField(max_length=25, null=True, blank=True)
  </code></pre>
  
  <p>And using the builtin UserAdmin</p>
  
  <pre><code>admin.site.register(User, UserAdmin)
  </code></pre>
  
  <p>While editing the user record works fine, but when I add a user, I get the following error</p>
  
  <pre><code>Exception Value: 
  relation "auth_user" does not exist
  LINE 1: ...user"."is_active", "auth_user"."date_joined" FROM "auth_user...
  </code></pre>