Add program times for items (Z#23178639)

* Add program times for items

* Fix frontend date validation

* Add ical data for program times [wip]

* Improve ical data for program times

* Remove duplicate code and add comments

* Adjust migration

* Remove program times form for event series

* Add pdf placeholder [wip]

* Improve explanation text with suggestion

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

* Fix import sorting

* Improve ical generation

* Improve ical entry description

* Fix migration

* Add copyability for program times fot items and events

* Update migration

* Add API endpoints/functions, fix isort

* Improve variable name

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* Remove todo comment

* Add documentation, Change endpoint name

* Change related name

* Remove unnecessary code block

* Add program times to item API

* Fix imports

* Add log text

* Use daterange helper

* Add and update API tests

* Add another API test

* Add program times to cloning tests

* Update query count because of program times query

* Invalidate cached tickets on program time changes

* Reduce invalidation calls

* Update migration after rebase

* Apply improvements to invalidation from review

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* remove unneccessary attr=item param

* remove unnecessary kwargs for formset_factory

* fix local var name being overwritten in for-loop

* fix empty formset being saved

* Use subevent if available

* make code less verbose

* remove double event-label in ical desc

* fix unnecessary var re-assign

* fix ev vs p.subevent

---------

Co-authored-by: Raphael Michel <michel@pretix.eu>
Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/base/models/items.py

@@ -2294,3 +2294,27 @@ class ItemVariationMetaValue(LoggedModel):
 
     class Meta:
         unique_together = ('variation', 'property')
+
+
+class ItemProgramTime(models.Model):
+    """
+    This model can be used to add a program time to an item.
+
+    :param item: The item the program time applies to
+    :type item: Item
+    :param start: The date and time this program time starts
+    :type start: datetime
+    :param end: The date and time this program time ends
+    :type end: datetime
+    """
+    item = models.ForeignKey('Item', related_name='program_times', on_delete=models.CASCADE)
+    start = models.DateTimeField(verbose_name=_("Start"))
+    end = models.DateTimeField(verbose_name=_("End"))
+
+    def clean(self):
+        self.clean_start_end(start=self.start, end=self.end)
+        super().clean()
+
+    def clean_start_end(self, start: datetime = None, end: datetime = None):
+        if start and end and start > end:
+            raise ValidationError(_("The program end must not be before the program start."))