Set secrets via Terraform's GitHub provider

[Kalmalyzer]

Today, the setup of a new project involves manually adding a number of secrets to the Game and Engine GitHub repos. We should get rid of those steps by using the GitHub provider.

Before we can do so reliably, https://github.com/terraform-providers/terraform-provider-github/issues/422 and https://github.com/terraform-providers/terraform-provider-github/issues/501 need to be resolved.

[Kalmalyzer]

I'm no longer convinced that this is a good idea. It would require the workflow to have a lot more permissions than today for GitHub. If anything scripted goes wrong or if outsiders somehow acquire those credentials, it can result in major havoc. Closing.