Add pilot from plane.watch/planefeeder

[mikenye]

This PR adds a very basic and work-in-progress web UI (named "pilot") that will configure network settings (for now).

Pilot can be built by changing into the pilot directory and running go build ./cmd/pilot.

There is a system service definition in pilot/pilot.service, which expects the pilot binary to be located at /opt/pilot/pilot.

Pilot expects that netplan is installed, and will use networkd as the renderer. It will create a netplan YAML format file at: /etc/netplan/pilot.yaml.

Pilot will listen on :80.

You can "skin" the web UI by editing the HTML template files located in pilot/lib/webui/*.html.

Currently if the user enters invalid values, pilot will simply return a HTTP error. Ideally, we should perform validation on the form using javascript, and also return any errors relating to user input back to the user.

I'll continue to give you more PRs as this is developed, and likewise would be happy to receive PRs from you if you work on it. :-)

[kevinelliott]

Not a bad first pass @mikenye ... I like that it's in Go (some kind of optimized static binary asset). I have some opinions that might impact in the future, but for now this is a nice addition to start moving us forward. Thank you!