Refactor the copyright checker

[jennyhickson]

The copyright checker script relies on many very specific templates to check against which is a maintenance problem. We've been discussing that it would be better to check for the fundamentals of what a copyright statement should say (e.g. "copyright" and "all rights reserved" etc) in order to determine that a statement of some sort is present without worrying about the details.

We can then trust reviewers to look at the full wording when a file is new or the statement is changed.