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.
I would still prefer that we keep all configurables in one place. Looking at this one file tells us that cache expiry times varies for different content types. If it is spread across controllers, we might not notice. When I came here to add an entry for document collections, I also learnt that we're doing something similar with uploads.