Test & evaluate GNOME Keysign

There’s a Debian RFP, but we could evaluate it before someone bothers packaging it.

• https://blogs.gnome.org/muelli/2015/03/dfn-workshop-2015/
• https://muelli.cryptobitch.de/paper/2015-casual-keysigning.pdf

Parent Task: #6338 (closed)

Related issues

• Blocks #8401 (closed)
• Blocked by #8402 (closed)

Original created by @intrigeri on 8400 (Redmine)