Discounts: Add applicability date ranges (Z#23151897) (#4927)

* Add event date fields, add preliminary range check

* Remove function, use filtered queryset for subevent id limit

* Improve and fix date range check

* Add formfields

* Add tests

* Improve tests

* Add new fields to API and documentation

* Add migration

* Change description according to suggestion

* Change discount apply signature, remove unnecessary query

* Rename new fields, simplify range check

* Rename fields in template

* Apply suggestions from code review

Co-authored-by: Raphael Michel <michel@rami.io>

---------

Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/base/migrations/0279_discount_event_date_from_discount_event_date_until.py

@@ -0,0 +1,23 @@
+# Generated by Django 4.2.19 on 2025-03-18 09:12
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0278_login_source_add_unique_together'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='discount',
+            name='subevent_date_from',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name='discount',
+            name='subevent_date_until',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+    ]