Improve GitHub connection experience

Improve the GitHub connection experience for Enterprise customers by supporting thousands instead of hundreds of repos and making it easier to add and delete repos to the connection.

Part 1, connection REST APIs

Provide REST endpoints so customers can build tooling to orchestrate connecting of GitHub repos to Azure DevOps projects. These APIs can be used at scale to connect hundreds or thousands of GitHub repos to a given project.

Part 2, UX improvements

Improve the user experience of connecting many GitHub repos to an Azure DevOps project.