auth: GitHub authentication fixes
Created by: tsenart
This PR fixes a regression introduced in #7331 and reported in #7518 (closed) as
well setting a well-typed default for the new allowOrgs
setting.
The issue was that oauth.getProvider
does a concrete type assertion
and in the previous PR we changed what was stored in as the interface
implementation to a wrapper type.
Fixes #7518 (closed)