Systemd service definition for Raintale

[shawnmjones]

Our existing start-gui.sh and stop-gui.sh scripts work well for Docker deployment, but, as noted by @ato, we need something more robust for general server installation.

@ato has provided a Gist here that provides this capability:
https://gist.github.com/ato/69fc6952ce57d329a402a6f9f128f6e1

We need to include these as separate files in the raintale/raintale-gui/ folder to be included in a future installer. Several people, including Olga from the IIPC, have asked that we attempt to make services like the Raintale GUI easier to install, and @ato's systemd work goes a long way to making this happen.

We will have to independently test this on another systemd-capable machine as well.