Delete training repos after a few months.

As a trainer
I would like GH to delete exiting repos a few months later than creation, aka training date.
So that if the same student goes to the same training later, the creation of new repos will not fail for the new training.

I had a training where a student had already a repo created. He did not have access to it as the invitation had expired already.
As I am not the creator I could not remove the repo to allow the script create the a new one for my training.

[collision repo](https://github.com/ps-developers-sandbox/github-games-JonLehne/issues?q=is%3Aissue+is%3Aopen+author%3Anptrsn)

CC: @mericstam