Add a simple test mode (#1181)

- [x] Provide data model and configuration toggle
- [x] Allow to delete individual test orders
- [x] Add tests
- [x] Add a prominent warning message to the backend if test mode orders exist (even though test mode is off), as this leads to wrong statistics
- [x] Decide if and how to generate invoices for test orders as invoice numbers cannot be repeated or should not have gaps.
- [x] Decide if and how we expose test orders through the API, since our difference pull mechanism relies on the fact that orders cannot be deleted.
- [x] Decide if and how we want to couple test modes of payment providers?
- [ ] pretix.eu: Ignore test orders for billing
- [ ] Adjust payment providers: Mollie, bitpay, cash, fakepayment, sepadebit

![download](https://user-images.githubusercontent.com/64280/53009081-fe420d80-343a-11e9-8361-b8511c988598.png)

src/pretix/base/invoice.py

@@ -9,7 +9,7 @@ import vat_moss.exchange_rates
 from django.contrib.staticfiles import finders
 from django.dispatch import receiver
 from django.utils.formats import date_format, localize
-from django.utils.translation import pgettext
+from django.utils.translation import pgettext, ugettext
 from PIL.Image import BICUBIC
 from reportlab.lib import pagesizes
 from reportlab.lib.enums import TA_LEFT
@@ -267,6 +267,13 @@ class ClassicInvoiceRenderer(BaseReportlabInvoiceRenderer):
         canvas.saveState()
         canvas.setFont(self.font_regular, 8)
 
+        if self.invoice.order.testmode:
+            canvas.saveState()
+            canvas.setFont('OpenSansBd', 30)
+            canvas.setFillColorRGB(32, 0, 0)
+            canvas.drawRightString(self.pagesize[0] - 20 * mm, (297 - 100) * mm, ugettext('TEST MODE'))
+            canvas.restoreState()
+
         for i, line in enumerate(self.invoice.footer_text.split('\n')[::-1]):
             canvas.drawCentredString(self.pagesize[0] / 2, 25 + (3.5 * i) * mm, line.strip())