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/plugins/sendmail/forms.py

@@ -1,15 +1,22 @@
 from django import forms
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import pgettext_lazy, ugettext_lazy as _
 from i18nfield.forms import I18nFormField, I18nTextarea, I18nTextInput
 
 from pretix.base.forms import PlaceholderValidator
 from pretix.base.models import Order
+from pretix.base.models.event import SubEvent
 
 
 class MailForm(forms.Form):
     sendto = forms.MultipleChoiceField()  # overridden later
     subject = forms.CharField(label=_("Subject"))
     message = forms.CharField(label=_("Message"))
+    subevent = forms.ModelChoiceField(
+        SubEvent.objects.none(),
+        label=_('Only send to customers of'),
+        required=False,
+        empty_label=pgettext_lazy('subevent', 'All dates')
+    )
 
     def __init__(self, *args, **kwargs):
         event = kwargs.pop('event')
@@ -35,3 +42,7 @@ class MailForm(forms.Form):
             label=_("Send to"), widget=forms.CheckboxSelectMultiple,
             choices=choices
         )
+        if event.has_subevents:
+            self.fields['subevent'].queryset = event.subevents.all()
+        else:
+            del self.fields['subevent']