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/migrations/0143_auto_20200210_1038.py

@@ -0,0 +1,42 @@
+# Generated by Django 2.2.8 on 2020-02-10 10:38
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+import pretix.base.models.base
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0142_auto_20191215_1522'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='ItemMetaProperty',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False)),
+                ('name', models.CharField(db_index=True, max_length=50)),
+                ('default', models.TextField()),
+                ('event', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='item_meta_properties', to='pretixbase.Event')),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=(models.Model, pretix.base.models.base.LoggingMixin),
+        ),
+        migrations.CreateModel(
+            name='ItemMetaValue',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False)),
+                ('value', models.TextField()),
+                ('item', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='meta_values', to='pretixbase.Item')),
+                ('property', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='item_values', to='pretixbase.ItemMetaProperty')),
+            ],
+            options={
+                'unique_together': {('item', 'property')},
+            },
+            bases=(models.Model, pretix.base.models.base.LoggingMixin),
+        ),
+    ]