Fix #499 -- Refactor paymentinfo to payment_info (#501)

* Refactor paymentinfo to payment_info, resolve #499

* Fix calling of object attribute on tuple

* Minor update to setup documentation

* Do not use short words for typochecks

* Text clarification

* Refactor paymentinfo to payment_info, resolve #499

* Include data migration for existing event settings, resolve #499

src/pretix/base/migrations/0061_auto_20170521_0942.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.7 on 2017-05-21 09:42
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+def rename_placeholder(app, schema_editor):
+    EventSettingsStore = app.get_model('pretixbase', 'Event_SettingsStore')
+
+    for setting in EventSettingsStore.objects.all():
+        if setting.key == 'mail_text_order_placed':
+            new_value = setting.value.replace('{paymentinfo}', '{payment_info}')
+            setting.value = new_value
+            setting.save()
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0060_auto_20170510_1027'),
+    ]
+
+    operations = [
+        migrations.RunPython(rename_placeholder)
+    ]