Implement GitHub authentication

[Greenheart]

This should connect a GitHub account (userId) to a user in our database, to allow us to track metadata for the change history.

With GitHub authentication in place, we should have some way to distinguish trusted users - essentially some kind of authorization, e.g. an role-based access control system where each account can be given a number of roles, and only for example users with the editor role may edit our climate data.

We also want to manage which users we trust as editors or viewers. This could be done via GitHub organization teams, to manage members. Then we could have a Editor team and an Viewer team (who can see non-public data).