Automate checking that PR authors have signed the Contributor License Agreement

[Jermolene]

Our contribution rules require authors of PRs to have signed the Contributor License Agreement.

Currently I manually check the CLA for unfamiliar contributors, which is obviously an error prone process, and mistakes do get made.

I would very much like some help setting up an automated bot to perform this task. It would run on PR creation, and add a comment if no CLA signature can be found:

Thank you for your contribution, and welcome to the project. We require that all project contributors sign our Contributors License Agreement to explicitly assign the copyright of their contribution to the project. Please make a separate PR targeting the tiddlywiki-com branch with your signature as described in contributing.md

The bot should also be re-run each time the PR is updated. If the CLA signature is found then the existing comment should be updated with a brief "Thank you for signing the Contributors License Agreement".

Would anyone be able to take on this task?

Questions and suggestions welcome.