Replace init script with systemd services

[mayankchhabra]

• systemd is the facto init system for Linux.
• Greater dependency control. For instance, we can now guarantee that Umbrel will NOT start if the external storage mounter fails for whatever reason.
• Faster boot time as we can run services in parallel without blocking the boot process.
• Choice of logging. For instance, we can now log umbrel-startup.service and umbrel-external-storage.service to syslog, and umbrel-connection-details service to tty.
• Overall more verbose, clean and straightforward than using rc.local + initd.