Refs #775 -- Pluggable authentication backends (#1447)

* Drag-and-drop: Force csrf_token to be present

* Rough design

* Missing file

* b.visble

* Forms

* Docs

* Tests

* Fix variable

src/pretix/control/views/organizer.py

@@ -20,6 +20,7 @@ from django.views.generic import (
 )
 
 from pretix.api.models import WebHook
+from pretix.base.auth import get_auth_backends
 from pretix.base.models import Device, Organizer, Team, TeamInvite, User
 from pretix.base.models.event import Event, EventMetaProperty
 from pretix.base.models.organizer import TeamAPIToken
@@ -602,6 +603,9 @@ class TeamMemberView(OrganizerDetailViewMixin, OrganizerPermissionRequiredMixin,
                 if self.object.invites.filter(email__iexact=self.add_form.cleaned_data['user']).exists():
                     messages.error(self.request, _('This user already has been invited for this team.'))
                     return self.get(request, *args, **kwargs)
+                if 'native' not in get_auth_backends():
+                    messages.error(self.request, _('Users need to have a pretix account before they can be invited.'))
+                    return self.get(request, *args, **kwargs)
 
                 invite = self.object.invites.create(email=self.add_form.cleaned_data['user'])
                 self._send_invite(invite)