Support different auth than GitHub

[coolya]

ktor supports various kinds or oauth2 backends. In order to support those we would need to be able to configure the backend to use with client secret etc.

We also need to abstract away getting the user info in the servers Auth.kt which right now calls GitHub API directly to get the the user info.

[coolya]

Ideally we want input from actual users what they would like to see as authentication backend for their private instances.