.deb packaging

This adds a preliminary .deb package.

What it does

• Installs signald
• Creates signald user and group
• Installs signald.service systemd unit file
• Adds ci config to ensure automated builds.

What it doesn't do (and ought to?)

• Configure signald using debconf.
• Create the signald user's home folder (maybe we should just move away from storing config in the home folder).
• Enable the systemd unit.
• Publish the package into a debian repo and ensure that repo is installed in the install scripts.