Fix #774 -- Make question options sortable (#786)

* add position field

* add question option sorting logic
* add meta class to question option for sorting
* regenerate migration
* add template content and view mechanics

* Rename migration after rebase & update dependency

src/pretix/base/migrations/0084_questionoption_position.py

@@ -0,0 +1,41 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.9 on 2018-03-03 16:41
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+def set_position(apps, schema_editor):
+    Question = apps.get_model('pretixbase', 'Question')
+    for q in Question.objects.all():
+        for i, option in enumerate(q.options.all()):
+            option.position = i
+            option.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0083_auto_20180228_2102'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='questionoption',
+            options={'ordering': ('position', 'id'), 'verbose_name': 'Question option', 'verbose_name_plural': 'Question options'},
+        ),
+        migrations.AddField(
+            model_name='questionoption',
+            name='position',
+            field=models.IntegerField(default=0),
+        ),
+        migrations.AlterField(
+            model_name='question',
+            name='position',
+            field=models.PositiveIntegerField(default=0, verbose_name='Position'),
+        ),
+        migrations.RunPython(
+            set_position, 
+            reverse_code=migrations.RunPython.noop,
+        ),
+    ]