Get a subscription for securiteinfo

Looking at the bounce reasons in the logs, we're still forwarding malware/phishing/etc. Most notably, riseup is picking up on crap we're not filtering out based on securiteinfo clamav signatures.

I'd like to have us import those signatures as well, but this requires a subscription. It's only $30 per year, I think that's well worth it. See also: https://www.securiteinfo.com/clamav-antivirus/improve-detection-rate-of-zero-day-malwares-for-clamav.shtml?lg=nl