Add fork-me banner for AGPL compliance by default

[nilbus]

AGPL requires that users hosting this app provide the source code to their version of app.

Provide a banner like this one with a link to the github repository.

My pinewood-derby project implements this, and it should be easy to rip out from there.

Ideally, it should detect the git remote URL and link to the fork (if github) rather than the parent project.