Support adding github teams as reviewers

[jerry-skydio]

Github supports having team as well as user reviewers. This would probably be the easiest to use if it just went on top of our existing Reviewers: syntax. We'd have to try to look up each input as both a team and a user , then add the list of team ids to a new input to the add reviewers mutation.

Adding or querying teams unfortunately requires a new permission for the access token, so we might want to hide the entire thing behind a flag to not break current users.