Simplify copyright information by maintaining copyright holders in a centralized location

Rather than specifying all of the individual copyright holders individually in each JS file, we should switch to using an authors/contributors/copyrights file that includes all of them and update the individual copyright statements to point at them.

see: 

• https://opensource.google.com/docs/releasing/authors/
• https://ben.balter.com/2015/06/03/copyright-notices-for-websites-and-open-source-projects/
• https://softwarefreedom.org/resources/2012/ManagingCopyrightInformation.html
• https://github.com/kentcdodds/all-contributors