Copyright Assignment Stuff

While working on !356 (closed), I realised that the CI approach to checking for assignment information will not work out.

So, we need to do that manually. But the scripts for that and the updated AUTHORS file are still useful to have.
Also, this MR adds a new script for sending the Copyright Assignment form emails to new contributors.

Approver's checklist:

• The author has submitted the FSF Copyright Assignment and is listed in AUTHORS
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code