Use OAuth for accessing private repos for deployment

While we will use OAuth for signing in, we would also like to use OAuth for deployments. To be polite and not scare away users, we do not plan to request permission to Github repos during login, so accessing repos will involve an additional OAuth request for permission to access these. Once granted, we can use the token/authority to query private repos for tags and to authorize deployments from those repos.

(This is not currently ready for UI work)