authz/github: fixes for oauth scopes check

Created by: bobheadxi

follow-up patches to https://github.com/sourcegraph/sourcegraph/pull/24328

• only show fetch error if scopes fetch fails
• use markdown code block for read:repo and other slight message tweaks
• add an oauth scope check that uses the root github API URL (https://docs.github.com/en/rest/reference/meta) in case token is not a user token
• pass on all allowed org scopes ([]string{"read:org", "write:org", "admin:org"})

most of diff is VCR update from integration test 😓