New latest changes endpoint

For the collections frontend latest content view, build a new endpoint in Collections API which returns the recently published or updated documents for a sub-topic, including their change notes when available.
https://www.pivotaltracker.com/story/show/81091510
I missed that.  Yeah it does, and is therefore missing the other bits of information needed to render the page.

@dp6ai and @pedrosmmoreira see https://github.com/alphagov/collections-api/blob/master/app/presenters/sector_presenter.rb#L18-L24