Send e-mails on order completion (#27)

src/pretix/presale/templates/pretixpresale/email/order_placed.txt

@@ -0,0 +1,13 @@
+{% load i18n %}{% blocktrans with event=event.name total=order.total|floatformat:2 currency=event.currency paymentinfo=payment date=order.expires|date url=url %}Hello,
+
+we successfully received your order for {{ event }} with a total value
+of {{ total }} {{ currency }}. Please complete your payment before {{ date }}.
+{{ paymentinfo }}
+
+You can view the status of your order at
+
+{{ url }}
+
+Best regards,
+Your {{ event }} team
+{% endblocktrans %}