Warn about expiring https certificates

[AuroraLS3]

I would like to be able to..

Warn about certificates that are about to expire or have expired

Is your feature request related to a problem? Please describe.

Let's encrypt certificates are valid for 90 days. It might be useful if Plan gave warning about their expiry before they do.

It might be complicated to do, but I think the certificates contain their validity date information and that should be accessible via Java. I think #1987 needs to be done before this one though.