Refactor cancelling positions and orders in the data model (#1088)

- [x] Data model
- [x] display in order view in backend
- [x] review all usages of OrderPositions.objects
- [x] review all usages of order.positions
- [x] review all other model usages
- [x] review plugins
- [x] plugins backwards-compatible API?
- [x] decide on way forward for REST API
- [x] need to cancel fees
- [x] tests
- [ ] plugins
  - [ ] gdpr
  - [ ] reports
- [x] docs

src/pretix/base/migrations/0104_auto_20181114_1526.py

@@ -0,0 +1,63 @@
+# Generated by Django 2.1.1 on 2018-11-14 15:26
+
+import django.db.models.deletion
+import django.db.models.manager
+import jsonfallback.fields
+from django.db import migrations, models
+
+
+def change_refunded_to_canceled(apps, schema_editor):
+    Order = apps.get_model('pretixbase', 'Order')
+    Order.objects.filter(status='r').update(status='c', total=0)
+    Order.objects.filter(status='c').update(total=0)
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0103_auto_20181121_1224'),
+    ]
+
+    operations = [
+        migrations.AlterModelManagers(
+            name='orderposition',
+            managers=[
+                ('all', django.db.models.manager.Manager()),
+            ],
+        ),
+        migrations.AddField(
+            model_name='orderposition',
+            name='canceled',
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AlterField(
+            model_name='orderfee',
+            name='fee_type',
+            field=models.CharField(choices=[('payment', 'Payment fee'), ('shipping', 'Shipping fee'), ('service', 'Service fee'), ('cancellation', 'Cancellation fee'), ('other', 'Other fees'), ('giftcard', 'Gift card')], max_length=100),
+        ),
+        migrations.AlterField(
+            model_name='orderposition',
+            name='order',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, related_name='all_positions',
+                                    to='pretixbase.Order', verbose_name='Order'),
+        ),
+        migrations.AlterModelManagers(
+            name='orderfee',
+            managers=[
+                ('all', django.db.models.manager.Manager()),
+            ],
+        ),
+        migrations.AddField(
+            model_name='orderfee',
+            name='canceled',
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AlterField(
+            model_name='orderfee',
+            name='order',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, related_name='all_fees', to='pretixbase.Order', verbose_name='Order'),
+        ),
+        migrations.RunPython(
+            change_refunded_to_canceled, migrations.RunPython.noop
+        )
+    ]