RFC 40: GraphQL APIs

Created by: unknwon

This issue tracks implementation of GraphQL APIs for RFC 40: Explicit Repository Permissions Model.

• Update schema.graphql
• setRepositoryPermissionsForUsers #7547
• authorizedUserRepositories #7610
• usersWithPendingPermissions #7678
• Repository.authorizedUsers
• Tests https://github.com/sourcegraph/sourcegraph/pull/6427#issuecomment-557392099