Add GitHub App that manages the Project

[obscurerichard]

We make heavy use of a GitHub Project but there's a significant limitation in the way Projects works, in that it will only auto-add issues from one of the repositories to the project when created.

We can fix this by making a GitHub app and associated infrastructure as code to maintain the project.

The GitHub app and app installation could be defined with infrastructure as code, maybe Terraform. The required webhook probably ought to be a Lambda function hooked up to API Gateway, this could be an AWS SAM app (these are pretty easy).

It could also be yet another container in the main Freezing Saddles docker compose setup. This is already running and is very lightly loaded...

The features could be:

• Add every issue and PR that pops up in a freezingsaddles/* repo to the project once it is created, if it is not already
• Assign a default milestone to every issue
• Be able to auto-add the default milestone to the project
• Maybe assign a label triage to any issue or PR that lacks issues when created