Collection cache max age should be 5m

www.agileplannerapp.com/boards/173808/cards/8023

HMRC need to publish a collection page which needs to reflect the latest changes in no longer than 5m.

made a change to the `CacheControlHelper` to allow overriding the `cache_max_age` from the controller, so that we can set a max age of 5m for collections.
Given that collections are only a single controller and this is used in one place besides tests I'm questioning the wisdom of extracting it out.