add US Public Domain URL to license field pull down

[rebeccawilliams]

See also: https://github.com/project-open-data/project-open-data.github.io/issues/453

[ykhadilkar]

@rebeccawilliams which one do you want to add?

• http://www.copyright.gov/title17/92chap1.html#105 (this is my preference, with my motivations being an authoritative permalink)
• http://www.usa.gov/copyright.shtml, or
• a new Project Open Data URL, similar to: https://project-open-data.cio.gov/unknown-license/

[rebeccawilliams]

This was URL decided upon (it redirects to the 2nd): http://www.usa.gov/publicdomain/label/1.0/

See: https://project-open-data.cio.gov/open-licenses/

[ykhadilkar]

Got it.