Make this a GitHub app

[asmeurer]

We should make this so that other projects can easily reuse it. This requires making it a GitHub app.

Here is some code for a similar bot that works as a GitHub app https://github.com/MeeseeksBox/MeeseeksDev/blob/34fa2c7d26a0e376b2ff92fbc36c69cfe2ebf187/meeseeksdev/meeseeksbox/utils.py#L81-L305.

Here are the API docs for GitHub apps https://developer.github.com/apps/

CC @Carreau