Support GitHub SSO at team-level

Created by: christinelovett

Feature request description

Customer has large GitHub organizations and would like more flexibility when designating who has access Sourcegraph. The current allowOrgs is not narrow enough for their org policies.

Is your feature request related to a problem? If so, please describe.

Rather than having to create new organizations specifically for Sourcegraph on GitHub, customer would like to simply select which existing GitHub teams have access to Sourcegraph in the site config.