Send notification to staff-users if a certificate which is in-use is expiring

[alextreme]

Discussed with @sjoerdie

Currently we have external (Uptimerobot) monitoring of our web-facing certificates. However for our DigiD/eHerkenning certificates we don't have additional monitoring in place.

It would be useful to automatically send a notification to staff-users if a certificate which is in-use is expiring within 30 days. This would be a weekly check (so you get bothered at most 4x).

This could be a management command or via some other mechanism, as long as it is easily automated.