[Tails-project] New facility: email meeting reminder (#7763)

Hi,

thanks to Muri who did the bulk of the work, we now have everything in
place to send automated email reminders, for example about recurring
meetings, in a way that can be managed collectively.

If your team could benefit from this, please let me know: I'm happy to
add new reminders.

For the curious, the entry point to the configuration and code is
https://tails.boum.org/contribute/working_together/roles/sysadmins/#meeting-reminder

Cheers,
--
intrigeri