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.
To play Devil's advocate, I'd say it's better to have it as a named thing that makes it clear we're treating document collections as special rather than setting it directly in the controller to 5.minutes, which ends up looking a bit more arbitrary.