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/static/pretixbase/js/reldate.js

@@ -0,0 +1,42 @@
+$(document).on('pretix:bind-forms', () => {
+	document.querySelectorAll('.reldatetime, .reldate').forEach(container => {
+		const groups = container.querySelectorAll('.radio')
+
+		groups.forEach(group => {
+			const referenceSelect = group.querySelector('select[data-relative-choice]')
+			const beforeAfterSelect = group.querySelector('select[data-relation-choice]')
+			if (!referenceSelect || !beforeAfterSelect) return
+
+			const beforeOption = beforeAfterSelect.querySelector('option[value="before"]')
+			const afterOption = beforeAfterSelect.querySelector('option[value="after"]')
+
+			const updateBeforeOption = () => {
+				let supportsBefore = referenceSelect.selectedOptions[0].hasAttribute('data-supports-before')
+				if (beforeOption) {
+					beforeOption.disabled = !beforeOption.disabled && !supportsBefore
+				}
+
+				let supportsAfter = referenceSelect.selectedOptions[0].hasAttribute('data-supports-after')
+				if (afterOption) {
+					afterOption.disabled = !afterOption.disabled && !supportsAfter
+				}
+
+				let dirty = false
+				if (beforeOption.disabled && beforeAfterSelect.value === 'before') {
+					beforeAfterSelect.value = 'after'
+					dirty = true
+				}
+				if (afterOption.disabled && beforeAfterSelect.value === 'after') {
+					beforeAfterSelect.value = 'before'
+					dirty = true
+				}
+
+				if (dirty) {
+					beforeAfterSelect.dispatchEvent(new Event('change', {bubbles: true}))
+				}
+			}
+			referenceSelect.addEventListener('change', updateBeforeOption)
+			updateBeforeOption()
+		})
+	})
+})