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/pretix/base/forms/widgets.py

@@ -314,3 +314,18 @@ class BusinessBooleanRadio(forms.RadioSelect):
             'False': False,
             False: False,
         }.get(value)
+
+
+class OptionAttrsSelect(forms.Select):
+    def __init__(self, *args, option_attrs=None, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.option_attrs = option_attrs or {}
+
+    def create_option(self, name, value, label, selected, index, subindex=None, attrs=None):
+        option = super().create_option(
+            name, value, label, selected, index, subindex=subindex, attrs=attrs
+        )
+        extra = self.option_attrs.get(str(value))
+        if extra:
+            option["attrs"].update(extra)
+        return option