Allow dependencies between questions (#1202)

- [x] data model
- [x] api
- [x] backend editor
- [x] backend validation logic
- [x] frontend display logic
- [x] frontend validation logic
- [x] test checkout step
- [x] test modify order in frontend
- [x] test modify order in backend
- [x] validation tests
- [x] correctly evaluate dependency tree in frontend?
- [x] copy events

src/pretix/base/migrations/0113_auto_20190312_0942.py

@@ -0,0 +1,27 @@
+# Generated by Django 2.1.5 on 2019-03-12 09:42
+
+import django.db.models.deletion
+import jsonfallback.fields
+from django.db import migrations, models
+
+import pretix.base.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0112_auto_20190304_1726'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='question',
+            name='dependency_question',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='dependent_questions', to='pretixbase.Question'),
+        ),
+        migrations.AddField(
+            model_name='question',
+            name='dependency_value',
+            field=models.TextField(blank=True, null=True),
+        ),
+    ]