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/base/pdf.py

@@ -431,6 +431,8 @@ class Renderer:
             return '(error)'
         if o['content'] == 'other':
             return o['text']
+        elif o['content'].startswith('itemmeta:'):
+            return op.item.meta_data.get(o['content'][9:]) or ''
         elif o['content'].startswith('meta:'):
             return ev.meta_data.get(o['content'][5:]) or ''
         elif o['content'] in self.variables: