Queueing and mapping utilities for outbound data sync (#4881)

Add a registry for datasync providers and an associated sync queue, to be used by 
plugins that transfer data from pretix orders to external systems. 
Additionally, provide a generic data mapping interface to be used in settings pages 
of such plugins, to let users configure which information from pretix to fill into
which data fields of the external system.

---------

Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/control/views/dashboards.py

@@ -383,6 +383,10 @@ def event_index(request, organizer, event):
     ctx['has_cancellation_requests'] = can_view_orders and CancellationRequest.objects.filter(
         order__event=request.event
     ).exists()
+    ctx['has_sync_problems'] = can_change_event_settings and request.event.queued_sync_jobs.filter(
+        Q(need_manual_retry__isnull=False)
+        | Q(failed_attempts__gt=0)
+    ).exists()
 
     ctx['timeline'] = [
         {