Use GitHub format Co-Authored-By for unlinked contributors

[desrosj]

Using GitHub flavored Co-Authored-By trailers with ID+username format emails will:

• Still allow parsing and conversion to w.org accounts.
• Still allows contributors to connect w.org accounts after a contribution is merged and receive credit.
• Gives contributors credit on GH for their contribution. With the current Unlinked contributors: x, y, z. format, they will never receive attribution on GH (only .org after their account is connected and the props script is run).

[desrosj]

After talking this over with several contributors and getting some input from @josephahaden, I've decided not to make this change.

Including attribution with a GitHub formatted email removes some of the incentive and motivation for a contributor to establish a connection between w.org and GH accounts.

However, I've decided to leave the Unlinked: x, y, z. format in place. It's reasonable to track the contribution in a way that allows us to attribute it to a contributor should they make a connection in the future.