Fixed #30451 -- Added ASGI handler and coroutine-safety.

This adds an ASGI handler, asgi.py file for the default project layout,
a few async utilities and adds async-safety to many parts of Django.

tests/async/tests.py

@@ -0,0 +1,36 @@
+from asgiref.sync import async_to_sync
+
+from django.core.exceptions import SynchronousOnlyOperation
+from django.test import SimpleTestCase
+from django.utils.asyncio import async_unsafe
+
+from .models import SimpleModel
+
+
+class DatabaseConnectionTest(SimpleTestCase):
+    """A database connection cannot be used in an async context."""
+    @async_to_sync
+    async def test_get_async_connection(self):
+        with self.assertRaises(SynchronousOnlyOperation):
+            list(SimpleModel.objects.all())
+
+
+class AsyncUnsafeTest(SimpleTestCase):
+    """
+    async_unsafe decorator should work correctly and returns the correct
+    message.
+    """
+    @async_unsafe
+    def dangerous_method(self):
+        return True
+
+    @async_to_sync
+    async def test_async_unsafe(self):
+        # async_unsafe decorator catches bad access and returns the right
+        # message.
+        msg = (
+            'You cannot call this from an async context - use a thread or '
+            'sync_to_async.'
+        )
+        with self.assertRaisesMessage(SynchronousOnlyOperation, msg):
+            self.dangerous_method()