do not allow unauthenticated access to unlink page (fixes #22405)

src/authentic2_auth_fc/views.py

@@ -488,6 +488,8 @@ class UnlinkView(LoggerMixin, FormView):
         return kwargs
 
     def dispatch(self, request, *args, **kwargs):
+        if not request.user.is_authenticated():
+            raise PermissionDenied()
         # We prevent unlinking if the user has no usable password and can't change it
         # because we assume that the password is the unique other mean of authentication
         # and unlinking would make the account unreachable.