Replace reboot-notifier cron email notification with an Icinga check

I’d love to receive less email, and to use have our monitoring dashboard be the place to go to know what we have to do on our systems. Also, the Icingaweb2 dashboard always knows what’s the current state of things, and has a concept of state transition, as opposed to a set of emails.

Related issues

• Related to #12455 (closed)

Original created by @intrigeri on 11598 (Redmine)