Change from rc.local to systemd service

[histefanhere]

Hi, amazing project! Thank you for the time and dedication you put into this, ~~and I hope you merge albums into main soon~~ :)

This PR changes the script to autostart via a systemd piink.service instead of a line in rc.local. Apart from the obvious like making it easier to see it's status via sudo systemctl status piink and to see live logs via sudo journalctl -f -u piink, it's much easier to start/stop manually which makes it nicer to switch between using this and ryanwa18/spotipi-eink.

Everything has been tested to start on boot, and it even removes the line from rc.local if it's there to support upgrading from previous installations. Thanks!