a8n: Handle GitHub review events when reviewer is team

Created by: mrnugget

This fixes https://github.com/sourcegraph/sourcegraph/issues/7715 by loading the reviewer even if it is a team instead of an actor. Previously the fields were blank because the GraphQL didn't return an Actor in the requestedReviewer field.

That caused multiple ChangesetEvents that wrap different github.ReviewRequestedEvents to have the same key, because their RequestedReviewer was empty.

To fix that this PR loads the requestedTeam and sets it on the github.ReviewRequestedEvent and github.ReviewRequestRemovedEvent if it is set instead of an actor.