Use Django's [mass mail method](https://docs.djangoproject.com/en/5.0/topics/email/#send-mass-mail) to optimize message sending in the admin 'check in all' routine. - `src/contestadmin/tasks.py: check_in_out_users`
Use Django's mass mail method to optimize message sending in the admin 'check in all' routine.
src/contestadmin/tasks.py: check_in_out_users