Allow to use a custom domain per event (#1617)

* Drop support for maindomain_urls/subdomain_urls in plugins

* Allow to use a custom domain per event

* Fix bug when manually saving domains

* Fix custom domains in debugging

* Fix middleware

* Fix middleware again, update docs

src/pretix/multidomain/migrations/0002_knowndomain_event.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.2.11 on 2020-03-20 14:05
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0146_giftcardtransaction_text'),
+        ('pretixmultidomain', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='knowndomain',
+            name='event',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.PROTECT, related_name='domains', to='pretixbase.Event'),
+        ),
+    ]