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
This should be "search".  https://github.com/alphagov/whitehall/blob/101e3d8b685203b68553f063a8b5ec18a4e77a37/lib/whitehall/search_index.rb#L21

Have you tried running it locally?  This should have exploded at that point.