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/settings.py

@@ -10,6 +10,8 @@ from hierarkey.models import GlobalSettingsBase, Hierarkey
 from i18nfield.strings import LazyI18nString
 from typing import Any
 
+from pretix.base.reldate import RelativeDateWrapper
+
 DEFAULTS = {
     'max_items_per_order': {
         'default': '10',
@@ -69,7 +71,7 @@ DEFAULTS = {
     },
     'payment_term_last': {
         'default': None,
-        'type': datetime,
+        'type': RelativeDateWrapper,
     },
     'payment_term_weekdays': {
         'default': 'True',
@@ -165,7 +167,7 @@ DEFAULTS = {
     },
     'ticket_download_date': {
         'default': None,
-        'type': datetime
+        'type': RelativeDateWrapper
     },
     'ticket_download_addons': {
         'default': 'False',
@@ -181,7 +183,7 @@ DEFAULTS = {
     },
     'last_order_modification_date': {
         'default': None,
-        'type': datetime
+        'type': RelativeDateWrapper
     },
     'cancel_allow_user': {
         'default': 'True',
@@ -437,6 +439,9 @@ def i18n_uns(v):
 settings_hierarkey.add_type(LazyI18nString,
                             serialize=lambda s: json.dumps(s.data),
                             unserialize=i18n_uns)
+settings_hierarkey.add_type(RelativeDateWrapper,
+                            serialize=lambda rdw: rdw.to_string(),
+                            unserialize=lambda s: RelativeDateWrapper.from_string(s))
 
 
 @settings_hierarkey.set_global(cache_namespace='global')