Alerts: enable/disable globally

As a scientist i would like to be able to disable alert sending temporarily, but retain all the settings, for example during shutdown.

## acceptance criteria

* Command to disable sending alerts, but remember which were enabled for when they are later restored

## Notes

See #5491 for background. This may be possible by either disabling the sending (so leaving everything else as is), or by stopping the db processing scan loop so they are not generated but all settings remain.