Each collection has an RSS 2.0 feed 

This is initially for editors picks (a selection of curated content selected daily for Google News) but it's written in such a way to provide any controller a way to generate RSS by passing a `Seq[model.trail]` to the object.

I decided to use the [Rome RSS library](https://github.com/rometools/rome) rather than a homebrew scala xml, or play view.xml template effort as these approaches are prone to invalid feeds in my experience.

@gklopper and I decided to add `webUrl` to the trail model. This should be used where the full, external, canonical URL is the only representation that matters (Eg, URIs, GUIDs etc.). In our case we never want the RSS link to point at a relative or host specific environment.

There's some utterances that we should not port over our full fat feeds, so I've left the complete article body out of the feed until we come to a decision on this.

The object outputs a string that can be sent straight to the client as `text/xml`, rather than a `scala.xml.NodeSeq`, even though the tests convert it back to XML.

The image follows the guidelines here, https://support.google.com/news/publisher/answer/1407682

all - we need to make a call to keep the r2 RSS convention of a _slash rss_ on the end of the URLs or move everything over to next-gen _dot json_. It feels wrong having both.

TODO - @janua I need help figuring out why the collection displayName doesn't appear.