Allow RelativeDate in relation to a moment of order (#6160)

* initial implementation

* factor out _resolve_base_date

* add js to prevent illegal inputs

* fix tests

* Update src/pretix/base/reldate.py

Co-authored-by: Raphael Michel <michel@pretix.eu>

* Apply suggestions from code review

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

* move js includes of to fragment_js.html

* add type annotations

* moves logic from RelativeDateWrapper into RelativeDate and adds BaseChoice for configuring which models attributes support which relationship

* fix tests

* test upgrade behaviour

* Apply suggestions from code review

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

* move reldate.js include in correct file

* add OptionAttrsSelect to allow select options with their own attributes per value

* add a little bit of information to the tests for future reference

* rewrite reldate.js use data-attributes

* general cleanup

* add test for order.subevents cases

* use correct choice format

* remove order.subevent variants

* various cleanup

* Apply suggestions from code review

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

* remove empty docstrings

* add depreciation warning

* change event listener to pretix:bind-forms

---------

Co-authored-by: Raphael Michel <michel@pretix.eu>
Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

src/tests/presale/test_orders.py

@@ -1076,7 +1076,7 @@ class OrdersTest(BaseOrdersTest):
         self.event.date_from = now() + datetime.timedelta(days=3)
         self.event.save()
         self.event.settings.set('ticket_download_date', RelativeDateWrapper(RelativeDate(
-            base_date_name='date_from', days=2, time=None, minutes=None
+            base_date_name='event__date_from', days=2, time=None, minutes=None
         )))
         response = self.client.post(
             '/%s/%s/order/%s/%s/download/%d/testdummy' % (self.orga.slug, self.event.slug, self.order.code,