Add meta_data for items (#1576)

* PoC for ItemMetaProperties/Values

* Missing is_valid

* ItemMetaProperties/Values in editable via API, cloneable

* Tests

* Add Docs

* Fix import order

* Fix another import sorting...

* Typeahead for ItemMetaValues

* Test for editing event-objects

* Fix typeahead permission checks

* Further access restriction

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

src/pretix/control/forms/event.py

@@ -1203,3 +1203,11 @@ QuickSetupProductFormSet = formset_factory(
     formset=BaseQuickSetupProductFormSet,
     can_order=False, can_delete=True, extra=0
 )
+
+
+class ItemMetaPropertyForm(forms.ModelForm):
+    class Meta:
+        fields = ['name', 'default']
+        widgets = {
+            'default': forms.TextInput()
+        }