Fucked Up Refs bot

Yet another of @nyanbinary@infosec.exchange s bots, this one as a bit of a compendium piece to me looking into CVE references ( https://blagh.nyanbinary.de/categories/cve-meta/ ).

This bot has not yet been fully implemented but two functions are planned:

* [Implemented] Check references newly added to CVEs for availability & post one that is (immediately) not available - expect this to run ~hourly but post rarely.
* [Planned] Check references in historical CVE records (limiting myself to those published in the last 3 years) on availability & post newly unavailable domains (2 strikes) - expect this to run ~weekly.