Fixed #19414 -- Added admin registration decorator

Thanks stavros for the suggestion.

django/contrib/admin/__init__.py

@@ -1,5 +1,6 @@
 # ACTION_CHECKBOX_NAME is unused, but should stay since its import from here
 # has been referenced in documentation.
+from django.contrib.admin.decorators import register
 from django.contrib.admin.helpers import ACTION_CHECKBOX_NAME
 from django.contrib.admin.options import ModelAdmin, HORIZONTAL, VERTICAL
 from django.contrib.admin.options import StackedInline, TabularInline

django/contrib/admin/decorators.py

@@ -0,0 +1,28 @@
+def register(*models, **kwargs):
+    """
+    Registers the given model(s) classes and wrapped ModelAdmin class with
+    admin site:
+
+    @register(Author)
+    class AuthorAdmin(admin.ModelAdmin):
+        pass
+
+    A kwarg of `site` can be passed as the admin site, otherwise the default
+    admin site will be used.
+    """
+    from django.contrib.admin import ModelAdmin
+    from django.contrib.admin.sites import site, AdminSite
+
+    def _model_admin_wrapper(admin_class):
+        admin_site = kwargs.pop('site', site)
+
+        if not isinstance(admin_site, AdminSite):
+            raise ValueError('site must subclass AdminSite')
+
+        if not issubclass(admin_class, ModelAdmin):
+            raise ValueError('Wrapped class must sublcass ModelAdmin.')
+
+        admin_site.register(models, admin_class=admin_class)
+
+        return admin_class
+    return _model_admin_wrapper