Setup signing server

We are automating more and more stuff (ex: arch-boxes#132 (closed)), so we need a signing server sooner rather than later.

Spec: https://gitlab.archlinux.org/archlinux/signstar

Alternatives:

• Fedora / Red Hat use https://pagure.io/sigul
• openSUSE uses https://github.com/openSUSE/obs-sign
• Mozilla uses XX
• https://github.com/coreos/fero