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/templates/pretixcontrol/pdf/index.html

@@ -344,6 +344,11 @@
                                         {% trans "Event attribute:" %} {{ p.name }}
                                     </option>
                                 {% endfor %}
+                                {% for p in request.event.item_meta_properties.all %}
+                                    <option value="itemmeta:{{ p.name }}">
+                                        {% trans "Item attribute:" %} {{ p.name }}
+                                    </option>
+                                {% endfor %}
                                 <option value="other">{% trans "Other…" %}</option>
                             </select>
                             <textarea type="text" value="" class="input-block-level form-control"