Convert Hoff to a Github App

[bertptrs]

Today for the very first time we ran out of API rate limit. While this is undoubtedly related to other uses of the account Hoff is using, we can side step the issue entirely and have Hoff be a Github app. This comes with a few advantages:

• A more uniform way to setup webhooks, as Hoff can request the right webhooks by itself
• Nicer access control
• A higher rate limit

It shouldn't change too much in a practical sense, though for one the authentication method for Git interaction would have to be changed from private key to token authentication. This is still supported, so I see no reason why this would be a problem.