Allow quotas to "close" when once full (#1344)

* Model

* Some UI

* API and logging

* Permission check

* Add tests

* Move option around

src/pretix/control/forms/item.py

@@ -169,7 +169,8 @@ class QuotaForm(I18nModelForm):
         fields = [
             'name',
             'size',
-            'subevent'
+            'subevent',
+            'close_when_sold_out'
         ]
         field_classes = {
             'subevent': SafeModelChoiceField,