Fixed #25855 -- Enhanced the migration warning for runserver.

Added unapplied migration count and the list of unmigrated apps.

tests/admin_scripts/another_app_waiting_migration/migrations/0001_initial.py

@@ -0,0 +1,21 @@
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Foo',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=255)),
+            ],
+        ),
+    ]

tests/admin_scripts/another_app_waiting_migration/models.py

@@ -0,0 +1,10 @@
+from __future__ import unicode_literals
+
+from django.db import models
+
+
+class Foo(models.Model):
+    name = models.CharField(max_length=255)
+
+    class Meta:
+        app_label = 'another_app_waiting_migration'