This systemd service unit may serve as an example for package
maintainers to write their own units or can be used as-is to run
listmonk on a operating system that uses systemd as its init system.
This is a template unit so that multiple listmonk instances can be
started and controlled through the @-syntax. Instances are started by
calling `systemctl start listmonk@myinstance.service` which goes on to
read `/etc/default/listmonk` and `/etc/default/listmonk-myinstance` as
environment files, uses `/etc/listmonk/myinstance.toml` as the
configuration file and creates a state directory in
`/var/lib/private/listmonk-myinstance`.
