Navigation and dashboard: Hide useless items (#5995)

* Navigation and dashboard: Hide useless items

If a user has access to *no organizer teams*, hide a number of things
from navigation and dashboard. This happens e.g. if a user only has
permissions in scope of the pretix-resellers or pretix-scheduling
plugins.

* New mechanism

src/pretix/base/models/auth.py

@@ -49,6 +49,7 @@ from django.core.exceptions import BadRequest, PermissionDenied
 from django.db import IntegrityError, models, transaction
 from django.db.models import Q
 from django.utils.crypto import get_random_string, salted_hmac
+from django.utils.functional import cached_property
 from django.utils.timezone import now
 from django.utils.translation import gettext_lazy as _
 from django_otp.models import Device
@@ -667,6 +668,11 @@ class User(AbstractBaseUser, PermissionsMixin, LoggingMixin):
         self.session_token = generate_session_token()
         self.save(update_fields=['session_token'])
 
+    @cached_property
+    @scopes_disabled()
+    def is_in_any_teams(self):
+        return self.teams.exists()
+
 
 class UserKnownLoginSource(models.Model):
     user = models.ForeignKey('User', on_delete=models.CASCADE, related_name="known_login_sources")