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/api/urls.py

@@ -112,6 +112,7 @@ item_router = routers.DefaultRouter()
 item_router.register(r'variations', item.ItemVariationViewSet)
 item_router.register(r'addons', item.ItemAddOnViewSet)
 item_router.register(r'bundles', item.ItemBundleViewSet)
+item_router.register(r'program_times', item.ItemProgramTimeViewSet)
 
 order_router = routers.DefaultRouter()
 order_router.register(r'payments', order.PaymentViewSet)