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
Slightly weird layout here, did you want
```
{
  details: {
    documents: [
      {
        latest_change_note: result[:latest_change_note],
        link: result[:link],
        timestamp: result[:public_timestamp],
        title: result[:title],
      }
    ]
  }
}
```