Add cache-control: private; for passworded pages
Add cache-control: private; for passworded pages

Force Akamai, but not downstream browser users, to not cache the HTTP
authenticated pages.

https://www.pivotaltracker.com/story/show/37375263

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet