Add "loginctl enable-linger" command to sidekiq systemd install task

[Paprikas]

Without lingering sidekiq systemd process will be shutdown.

The systemd user instance is started after the first login of a user and killed after the last session of the user is closed. Sometimes it may be useful to start it right after boot, and keep the systemd user instance running after the last session closes, for instance to have some user process running without any open session. Lingering is used to that effect.

From https://wiki.archlinux.org/index.php/Systemd/User

By default loginctl enable-linger enables lingering for current user. We can add variable if needed.

[seuros]

let add a setting for that to enable it and document it.

[Paprikas]

@seuros bump