events: add missing migration

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

authentik/events/migrations/0019_alter_notificationtransport_webhook_url.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.7 on 2021-10-04 15:31
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("authentik_events", "0018_auto_20210911_2217"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="notificationtransport",
+            name="webhook_url",
+            field=models.TextField(blank=True, validators=[django.core.validators.URLValidator()]),
+        ),
+    ]