* Upgrade Django to 3.0 and other dependencies to recent versions
* Fix otp version contsraint
* Remove six dependency
* Resolve some warnings
* Fix failing tests
* Update django-countries
* Resolve all RemovedInDjango31Warnings in test suite
* Run isort
* Fix import
* Update PostgreSQL version on travis
* Always call render_invoice_text - even if order is already paid
* Print PayPal payment ID on invoice if available and invoice is paid
* Also display Sale ID on invoice
* try/except for paymentId/SaleId
- [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
* Add data shredders for PII
* First working shredder
* Add more shredders
* Add new shredders and download confirmation
* tmp
* PayPal, Stripe, banktransfer
* Add icon to logs
* Untested payment log shredders
* Add waiting list shredder
* First tests
* Add tests for shredders
* Improve templats, link to shredder
* Test payment info shredders
* More tests
* Documentation
* Fix enabled flag in payment provider overview
* Fix minor issues
* Refs #99 -- Fix stripe support for zero-decimal currencies
* Add new money formatting method
* Force decimal places in many places
* Locale-aware currency rendering
* Fix currencies in more places
* More currency fixes
