Support Pipelines App with GitHub Enterprise

The integration between Azure Pipelines and GitHub is facilitated by a GitHub App. This app makes it easy to set up pipelines for a GitHub repository and to deliver events (CI, PR, Comment, etc) from GitHub to Azure Pipelines. The app also enhances the security of pull requests using comment triggers.

However, this app does not work when using Azure Pipelines with GitHub Enterprise. The goal of this work is to fix that.