Fixed #24100 -- Made the migration signals dispatch its plan and apps.

Thanks Markus for your contribution and Tim for your review.

tests/migrate_signals/custom_migrations/0001_initial.py

@@ -0,0 +1,16 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    operations = [
+        migrations.CreateModel(
+            "Signal",
+            [
+                ("id", models.AutoField(primary_key=True)),
+            ],
+        ),
+    ]