Ensure atom change feeds are rendered

This change is for https://www.pivotaltracker.com/story/show/63536630

The error was introduced here https://github.com/alphagov/trade-tariff-frontend/commit/15ff22b902a2df047ab6b81121e2e7a679be6509 sorry. Basically I restricted certain routes to only return HTML and therefore they don't render atom feeds now. More changes:

* Renders the feeds even atom format is not supplied (atom set as default)
* Added higher level specs that hit all the stack. Controller level ones didn't indicate there was an issue.