Add script/ci pipeline for key expiration checking

This script will return 1 if any user keys are expiring within two months' time. This was added in a separate contrib/ directory rather than including as a Makefile target because the shell would otherwise get quite messy-looking for Make-isms.

The CI pipeline has also been altered so that the routine checks for keys will only run on an automated pipeline and will be ignored for regular checks since we don't want development to be stalled due to someone's key expiring.

Fixes #4