2nd pass at consolidating license info + adding PD URL

[rebeccawilliams]

Added US Public Domain to the top of page and highlighted Worldwide PD options under Open Licenses. This eliminates repeating URLs several times.

[philipashlock]

I made some updates to simplify the different sections, to better differentiate the default status of U.S. Government Works from contractor work, and also to include CC0 as something that can optionally be applied to U.S. Government Works

This can be previewed at: https://jekit.codeforamerica.org/project-open-data/project-open-data.github.io/rebeccawilliams-licenseupdate2/open-licenses/

[jlberryhill]

+1

[JoshData]

Brilliant!