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/control/test_events.py

@@ -481,7 +481,7 @@ class EventsTest(SoupTest):
             'payment_term_last_0': 'absolute',
             'payment_term_last_1': (self.event1.presale_end - datetime.timedelta(1)).strftime('%Y-%m-%d'),
             'payment_term_last_2': '0',
-            'payment_term_last_3': 'date_from',
+            'payment_term_last_3': 'event__date_from',
             'tax_rule_payment': 'default',
         })
         assert doc.select('.alert-danger')
@@ -496,7 +496,7 @@ class EventsTest(SoupTest):
             'payment_term_last_0': 'relative',
             'payment_term_last_1': '',
             'payment_term_last_2': '10',
-            'payment_term_last_3': 'date_from',
+            'payment_term_last_3': 'event__date_from',
             'tax_rule_payment': 'default',
         })
         assert doc.select('.alert-danger')