Introduce Seat.sorting_rank (#1499)

* Introduce Seat.sorting_rank

* Fix comments

* Comments, for real

src/pretix/base/migrations/0141_seat_sorting_rank.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.4 on 2019-11-22 11:42
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0140_voucher_seat'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='seat',
+            name='sorting_rank',
+            field=models.BigIntegerField(default=0),
+        ),
+    ]