Fix #1001 -- Add product bundles (#1041)

* Data model + Editor

* Cart and order management

* Rebase migrations

* Fix typos, add tests on cart handling

* Add tests for checkout and quotas

* Add API endpoints

* Validation of settings

* Front page tax display

* Voucher handling

* Widget foo

* Show correct net pricing

* Front page tests

* reverse charge foo

* Allow to require bundling

* Fix test failure on postgres

src/pretix/base/migrations/0114_auto_20190316_1014.py

@@ -0,0 +1,60 @@
+# Generated by Django 2.1.7 on 2019-03-16 10:14
+
+import django.db.models.deletion
+import jsonfallback.fields
+from django.db import migrations, models
+
+import pretix.base.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0113_auto_20190312_0942'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='ItemBundle',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('count', models.PositiveIntegerField(default=1, verbose_name='Number')),
+                ('designated_price', models.DecimalField(blank=True, decimal_places=2, help_text="If set, it will be shown that this bundled item is responsible for the given value of the total price. This might be important in cases of mixed taxation, but can be kept blank otherwise. This value will NOT be added to the base item's price.", max_digits=10, null=True, verbose_name='Designated price part')),
+            ],
+        ),
+        migrations.AddField(
+            model_name='cartposition',
+            name='is_bundled',
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AlterField(
+            model_name='cartposition',
+            name='addon_to',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, related_name='addons', to='pretixbase.CartPosition'),
+        ),
+        migrations.AlterField(
+            model_name='orderposition',
+            name='addon_to',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, related_name='addons', to='pretixbase.OrderPosition'),
+        ),
+        migrations.AddField(
+            model_name='itembundle',
+            name='base_item',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='bundles', to='pretixbase.Item'),
+        ),
+        migrations.AddField(
+            model_name='itembundle',
+            name='bundled_item',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='bundled_with', to='pretixbase.Item', verbose_name='Bundled item'),
+        ),
+        migrations.AddField(
+            model_name='itembundle',
+            name='bundled_variation',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='bundled_with', to='pretixbase.ItemVariation', verbose_name='Bundled variation'),
+        ),
+        migrations.AddField(
+            model_name='item',
+            name='require_bundling',
+            field=models.BooleanField(default=False, help_text='If this option is set, the product will only be sold as part of bundle products.', verbose_name='Only sell this product as part of a bundle'),
+        ),
+    ]