Allow to hide a product unless a specific quota is sold out (#1351)

* Allow to hide a product unless a specific quota is sold out

* Fix required property

* Add API property and copy between events

src/pretix/base/migrations/0129_auto_20190724_1548.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.1 on 2019-07-24 15:48
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+import pretix.base.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0128_auto_20190715_1510'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='item',
+            name='hidden_if_available',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, to='pretixbase.Quota'),
+        ),
+    ]