Fix #467 -- Pluggable email placeholders (#1429)

* Fix #467 -- Pluggable email placeholders

* Previews

* Polishing

* Fix tests

* Add missing doc file

src/pretix/base/migrations/0135_auto_20191007_0803.py

@@ -0,0 +1,27 @@
+# Generated by Django 2.2.4 on 2019-10-07 08:03
+from django.core.cache import cache
+from django.db import migrations
+
+
+def mail_migrator(app, schema_editor):
+    Event_SettingsStore = app.get_model('pretixbase', 'Event_SettingsStore')
+
+    for ss in Event_SettingsStore.objects.filter(
+        key__in=['mail_text_order_approved', 'mail_text_order_placed', 'mail_text_order_placed_require_approval']
+    ):
+        chgd = ss.value.replace("{date}", "{expire_date}")
+        if chgd != ss.value:
+            ss.value = chgd
+            ss.save()
+            cache.delete('hierarkey_{}_{}'.format('event', ss.object_id))
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0134_auto_20190909_1042'),
+    ]
+
+    operations = [
+        migrations.RunPython(mail_migrator, migrations.RunPython.noop)
+    ]