Send signal on checkin (#1546)

* Send signal when orderposition is checked in

* Add position_checked_in signal to documentation

* Rename signal to checkin_created

* Update general.rst

* Update signals.py

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

src/pretix/control/views/checkin.py

@@ -14,6 +14,7 @@ from pytz import UTC
 from pretix.base.channels import get_all_sales_channels
 from pretix.base.models import Checkin, Order, OrderPosition
 from pretix.base.models.checkin import CheckinList
+from pretix.base.signals import checkin_created
 from pretix.control.forms.checkin import CheckinListForm
 from pretix.control.forms.filter import CheckInFilterForm
 from pretix.control.permissions import EventPermissionRequiredMixin
@@ -124,6 +125,7 @@ class CheckInListShow(EventPermissionRequiredMixin, PaginationMixin, ListView):
                         'list': self.list.pk,
                         'web': True
                     }, user=request.user)
+                    checkin_created.send(op.order.event, checkin=ci)
 
             messages.success(request, _('The selected tickets have been marked as checked in.'))