Add sales channels (#1103)

- [x] Data model
- [x] Enforce constraint
- [x] Filter order list
- [x] Set channel on created order
- [x] Products API
- [x] Order API
- [x] Tests
- [x] Filter reports
- [x] Resellers
- [ ] deploy plugins
  - [ ] posbackend
  - [ ] resellers
  - [ ] reports
- [x] Ticketlayouts
- [x] Support in pretixPOS

src/pretix/base/migrations/0103_auto_20181121_1224.py

@@ -0,0 +1,29 @@
+# Generated by Django 2.1.1 on 2018-11-21 12:24
+
+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', '0102_auto_20181017_0024'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='item',
+            name='sales_channels',
+            field=pretix.base.models.fields.MultiStringField(default=['web'], verbose_name='Sales channels'),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='order',
+            name='sales_channel',
+            field=models.CharField(default='web', max_length=190),
+            preserve_default=False,
+        ),
+    ]