auth: Prompt and allow re-authenticating with GitHub
Created by: ryanslade
This change will prompt a user who has been opted in to private code after already being part of the public code trial. In this case, we need to prompt them to re-authenticate with GitHub to request the additional repo
scope.
This change only updates the code host page and does not yet add a global banner.
https://user-images.githubusercontent.com/25610/119990169-4eb85600-bfc8-11eb-9409-90421aa9fe1e.mov
Part of https://github.com/sourcegraph/sourcegraph/issues/20229