api: optimize accessor
api: optimize accessor

concourse/concourse#5912

Compute teamRoles and isAdmin just once on initialization

Signed-off-by: Aidan Oldershaw <30ef7a6e0d016bc846f26d89dacbd23aceded206@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse