Add sub-events and relative date settings (#503)

* Data model

* little crud

* SubEventItemForm etc

* Drop SubEventItem.active, quota editor

* Fix failing tests

* First frontend stuff

* Addons form stuff

* Quota calculation

* net price display on EventIndex

* Add tests, solve some bugs

* Correct quota selection in more places, consolidate pricing logic

* Fix failing quota tests

* Fix TypeError

* Add tests for checkout

* Fixed a bug in QuotaForm

* Prevent immutable cart if a quota was removed from an item

* Add tests for pricing

* Handle waiting list

* Filter in check-in list

* Fixed import lost in rebase

* Fix waiting list widget

* Voucher management

* Voucher redemption

* Fix broken tests

* Add subevents to OrderChangeManager

* Create a subevent during event creation

* Fix bulk voucher creation

* Introduce subevent.active

* Copy from for subevents

* Show active in list

* ICal download for subevents

* Check start and end of presale

* Failing tests / show cart logic

* Test

* Rebase migrations

* REST API integration of sub-events

* Integrate quota calculation into the traditional quota form

* Make subevent argument to add_position optional

* Log-display foo

* pretixdroid and subevents

* Filter by subevent

* Add more tests

* Some mor tests

* Rebase fixes

* More tests

* Relative dates

* Restrict selection in relative datetime widgets

* Filter subevent list

* Re-label has_subevents

* Rebase fixes, subevents in calendar view

* Performance and caching issues

* Refactor calendar templates

* Permission tests

* Calendar fixes and month selection

* subevent selection

* Rename subevents to dates

* Add tests for calendar views

src/pretix/base/services/pricing.py

@@ -0,0 +1,33 @@
+from decimal import Decimal
+
+from pretix.base.decimal import round_decimal
+from pretix.base.models import Item, ItemVariation, Voucher
+from pretix.base.models.event import SubEvent
+
+
+def get_price(item: Item, variation: ItemVariation = None,
+              voucher: Voucher = None, custom_price: Decimal = None,
+              subevent: SubEvent = None, custom_price_is_net: bool = False):
+    price = item.default_price
+    if subevent and item.pk in subevent.item_price_overrides:
+        price = subevent.item_price_overrides[item.pk]
+
+    if variation is not None:
+        if variation.default_price is not None:
+            price = variation.default_price
+        if subevent and variation.pk in subevent.var_price_overrides:
+            price = subevent.var_price_overrides[variation.pk]
+
+    if voucher:
+        price = voucher.calculate_price(price)
+
+    if item.free_price and custom_price is not None and custom_price != "":
+        if not isinstance(custom_price, Decimal):
+            custom_price = Decimal(str(custom_price).replace(",", "."))
+        if custom_price > 100000000:
+            raise ValueError('price_too_high')
+        if custom_price_is_net:
+            custom_price = round_decimal(custom_price * (100 + item.tax_rate) / 100)
+        price = max(custom_price, price)
+
+    return price