Refactor authentication ActionBuilders [#MEM-403]

The basic cookie-based authentication is now wrapped by Play's
AuthenticatedBuilder rather than our own code, and the
authentication-backed-by-Identity-API lookup is now composed with this and
an ActionRefiner that does the API-lookup.

AuthActionTest removed because it's effectively testing
AuthenticatedBuilder now, which is tested within Play:

https://github.com/playframework/playframework/blob/9b33c199a/framework/src/play-integration-test/src/test/scala/play/it/auth/SecuritySpec.scala#L15

Play's AuthenticatedBuilder is also used elsewhere in the Frontend project
for the Admin app:

https://github.com/guardian/frontend/blob/3ee4896/admin/app/controllers/IndexController.scala#L17

https://jira.gutools.co.uk/browse/MEM-403