Introduce cancellation requests (#1627)

* Allow to adjust the cancellation fee without JS

* Introduce cancellation requests

* ignore→delete

* Change a few things after Martin's review

* Add a few tests

src/pretix/base/migrations/0148_cancellationrequest.py

@@ -0,0 +1,24 @@
+# Generated by Django 3.0.4 on 2020-03-25 10:05
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0147_user_session_token'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='CancellationRequest',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False)),
+                ('created', models.DateTimeField(auto_now_add=True)),
+                ('cancellation_fee', models.DecimalField(decimal_places=2, max_digits=10)),
+                ('refund_as_giftcard', models.BooleanField(default=False)),
+                ('order', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='cancellation_requests', to='pretixbase.Order')),
+            ],
+        ),
+    ]