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/tests/api/conftest.py

@@ -38,6 +38,7 @@ def event(organizer, meta_prop):
         is_public=True
     )
     e.meta_values.create(property=meta_prop, value="Conference")
+    e.item_meta_properties.create(name="day", default="Monday")
     e.settings.timezone = 'Europe/Berlin'
     return e