Check for expired/expiring keys

[sumnerwarren]

This PR creates a GitHub Action that checks a repository's blackbox keyring for expired or expiring (in one week) keys and creates an issue for each one that it finds.

You can see how to use it here (though I envision this being triggered via a schedule instead of pushes) and you can see created/updated issues in that repository as well.

[ekasbohm]

What happens when a key expires again? Is the same issue reopened, or is a new one created? Or would it depend whether the previous issue was closed? Could this action automatically close the issue if there's no longer an expired/expiring key for the user?

[sumnerwarren]

It's only looking at open issues. So if an issue were still open when a key expired then a new one would not be created (and I would argue this isn't doing it's job!). If a previous issue had been closed, then a new issue would be created.

It's only querying the first 10 open issues right now under two assumptions:

1. that these will be dealt with in a somewhat timely manner and not be left open forever
2. that we are using JIRA (ugh, why) for most work tracking and thus there shouldn't be many open issues in any given repo

Yea, we could add the opposite check: loop through open issues and close them if the key has been updated. My hope is that devs are referencing and closing the issues when the keys are updated via PR, but it wouldn't hurt to have the extra check. I created #2 to track this.